While you need to collect all 30 COG Tags to gain all three achievements, they do not all have to be found in one game. For example, if you found about 15 on casual and are beginning a game on hardcore or insane, what you collect on the other difficulty levels is cumulative.
Also, it doesn't matter if you play solo or on-line co-op, you'll still get credit for the tags you need to pick up if found by you or your partner. Once you (or you nad your partner have a particular tag, it will no longer be seen on that profile / save)
The most simple concept is to know to look for the COG Tags. Generally, COG tags are located near a Crimson Emblem. Whenever you see Omen, there is a COG Tag near it. Sometimes the tag will be literally under the Gears logo, other times you'll need to scout the area a bit to actually find the tag.
Here is a list of the locations of the COG Tags in Gears of War, listed by Act, Chapter, and COG Tag number.
COG Tag 1Edit
When you begin the game you choose a path, left or right (combat or training). Regardless of the path chosen, you'll see a Gears logo straight ahead, just under it is the first COG tag. Look towards the rear of the courtyard across from the cell area if you have trouble finding this right away.
COG Tag 2Edit
This tag is located in the first room where the door is cut open by the enemy locusts. It is in the rear of the room, under the stairway that you came down if you took the training route. You'll see a Gears logo on the wall, the COG Tag is on the floor a bit to the left of the logo.
COG Tag 3Edit
This particular tag's presence is not highlighted particularly well by a Gears logo, but it is still easy to find. As soon as you enter into the courtyard area you'll see some stairs to your left, go up them and turn left at the top and you should see the COG tags. Be advised that if you leave it too long to get the tags Dom will wander ahead and activate the end of the level, so if your playing on casual or can use the cover wall run straight to this tag before fighting in the courtyard.
COG Tag 4Edit
You don't really need help finding this COG Tag, as one of your comrades will point it out to you. In case you didn't hear him, it is in the first small courtyard of the level, near a short wall in the yards center and to the right of the Gears logo.
COG Tag 5Edit
In this area you'll encounter two emergence holes within a building (just after crossing the bridge). The COG Tags are near the second emergence hole, slightly hidden by some weeds, to the left of the door leading outside. Be sure to grab it before you head towards the streets. If you have already grabbed tag 4 in a previous playthrough an ally will point this one out instead.
COG Tag 6Edit
After fighting in the streets against troika gunners, and dealing with four emergence holes, you'll find this COG tag in the far left corner, located near some ammunition on the ground. It's in the only corner in which an emergence hole doesn't appear.
COG Tag 7Edit
Before you begin battle in this area take notice to the white rusty van with the Gears logo on it. Go immediately left of the van follow and its down in the corner.
COG Tag 8Edit
This COG tag is located in the hallway where Jack is called upon to open the door. You remember, the annoying hallways with a couple dozen Wretches! Anyhow, I suggest waiting until the Wretches have been defeated to collect the tag. The tag is to the far left of the door Jack is opening (if you're watching him open it). It's on the floor just next to a coffin looking figure made from cement. If you're quick enough you can get to it before the cutscene activates.
COG Tag 9Edit
This COG tag is located in the same room where you and the team discover that Rojas has been killed. As soon as you enter the room (by descending the stairs) the tag is hidden partially behind a pillar and in much of the shadows. While you're listening to the cut-scene head towards that pillar and grab the COG Tag.
COG Tag 10Edit
After you witness the brutal slaying of Gyules by a hungry, and somewhat annoyed Berserker, you can go grab his COG Tag. Just head to where he was killed, the COG tag is on the ground of the dead end area near some flames and it's next to Gyules neck.
COG Tag 11Edit
As you are trying to lure the Berserker outside, the Berserker will smash open three doors for you. As the third door is opened, you'll notice before you exit the enclosed area that to the left of that door, in an alcove is another set of COG Tags.
COG Tag 12Edit
Once you have lured the Berserker outside, look for the Gears logo on the wall. If you head left after you exit the building and go along the curved section of the wall you will find the COG Tag on the ground by the main wall, just after the Gears logo. the gears logo is hidden by green vines - sometimes miss cause of dealing with the berserker. The Gear is about 3/4 of the way to the back wall behind a pillar
COG Tag 13Edit
This is in the area right after you have regrouped. You'll find a small room with two sets of stairways and a couch. Open fire on the couch to destroy it and reveal the COG Tag hidden beneath it.
COG Tag 14
Once you exit the room with the square shaped catwalk, descend the stairs and go to the back of the alley the stairs are in. The COG tags are located at the end of the alley behind some rocks.
COG Tag 15Edit
As you near the Stranded Settlement, you'll see this set of COG Tags near a couple of newspaper vending machines between living trees. This is to the left of the settlement gate.
COG Tag 16Edit
As you make your way to checkpoint 2 the road curves off to the right. Before heading in that direction look to the left for a propane tank and a shed. Shoot the propane tank and with this new lighting you'll be able to enter the shed, where you'll find COG Tag 16.
COG Tag 17Edit
This COG Tag is located in the area where you guide Dom with the spotlight. As you enter the area, the Gears logo is on the wall behind and to your right but the COG Tag is to the left, in a darkened area near some rubble. To get this one, guide Dom down the street with the spotlight, then move the spotlight into position to illuminate that far left area were the tag is. When you exit the building, grab the tag before you meet back up with Dom. Alternatively, if you're quick enough you can run over there and grab it before the Kryll get you
COG Tag 18Edit
This COG Tag is located in the area where you shoot the propane tank out in the abandoned car and run down the road using the car as a torch. As soon as you enter the area thus exiting the building (where you'll see the car in front of you) look on the ground to your right around the edge of the building to locate the tag. You do not need the extra light from the propane tank in the car to get to it safely.
COG Tag 19Edit
This is in the rainy area outside. There are two buildings, one with a generator in it and another with an elevator inside. You'll come up on the one with the elevator first, they're on the right. Behind that building, towards the sea, is a long jetty, follow the jetty to the end to discover the COG Tag.
COG Tag 20Edit
This COG Tag is located in the room that has the rotting wooden floor boards that you will fall through. It's in the far right corner of the room. If you fall through before you find it, just head to the ladder in the corner and repeat until you get the COG Tag.
COG Tag 21Edit
This COG Tag is located downstairs after you exit the cart control room. Head down the stairs, toward the cars, and turn around. The COG Tags are to the right of the stairs you just came down.
COG Tag 22Edit
After you ride the drilling platforms down, stick to the right. You'll see a Gears logo, near is a grenade pickup and some COG tags.
COG Tag 23Edit
On the path along the right side you will see a building with a Gears logo. After some battle, you can head near the logo and pick up the COG tags to the right of the stairs and platform.
COG Tag 24Edit
Shortly after getting COG Tag 23, you will choose a path, battle, and then reunite. You'll then fight your way through a narrow area. This area has a lot of cover, before you leave the area look to the right, on the narrow ledge overlooking the pool of liquid you'll find this set of COG Tags. Grab them before heading to eliminate the Corpser.
COG Tag 25Edit
As soon as you enter the Campus Grinder area look to your right. You'll find this set of COG Tags near the second staircase on your right side.
COG Tag 26Edit
Straight after the cut-scene where you see a guy spewing up blood (the group of Gears being searched by stranded). You'll see a second rusted car up ahead in this area. The COG Tags are behind that car on the sidewalk.
COG Tag 27Edit
When you are in the Fenix Estate enter the room that is oval shaped on the one side, it's a library. In this room you'll find a desk, destroy it to reveal the COG tags. In fact, destroy other things while in this building, sometimes it leads to ammo or grenades. If you're not sure what room, it's the one where you get ambushed by Wretches coming up from the basement.
(PC VERSION ONLY) When you exit into a garden area with a half-open gate opposite where you came in. Behind it is a COG tag.
(PC VERSION ONLY) As you exit the dark underground car parking lot at the beginning of the chapter there's a stairwell. Instead of going up the right staircase, go down the left staircase to find the COG tag next to some flames.
(PC VERSION ONLY) When you fight the Brumak, there is a COG tag hidden at the far end of the arena near a broken cooker flanked by four thin concrete cover points.
COG Tag 28Edit
This COG tag is located on the first passenger car, as soon as you enter, to the left. This is just after Jack rips open the door.
COG Tag 29Edit
Just before you climb the ladder to exit to the third passenger car's roof, stay in the carriage area and search to the right of the carriage and you'll see the COG Tags on the ground next to a wall, be sure to pick them up.
COG Tag 30Edit
This COG Tag is located on the fifth passenger car, in the cabin area. Once you release the cylinders, go forward and enter the interior to grab the tags and some ammo.
Finding all the COG Tags in Gears of War will unlock the following achievements:
Time to Remember - 10 points: Recover one-third of the COG tags (on any difficulty) Honor-Bound - 20 points: Recover two-thirds of the COG tags (on any difficulty) For the Fallen - 30 points: Recover all of the COG tags (on any difficulty)