If you see the Gears logo (red skull and gear), it indicates that there is a C.O.G. tag somewhere nearby or in the same area, but not necessarily directly under it. The following is a list of areas followed by tags and their locations:
Tag #1 - After you choose paths go straight ahead. The tag is under the logo.
Tag #2 - In the room where the Locust cut through the door. If you are facing away from the door it is to the back of the right side.
Tag #3 - When you enter the outside courtyard, go up the small staircase on your left. It is in the corner on your left.
Trial By Fire
Tag #4 - It is in the small courtyard after you exit the building. It is to the right of the Gears logo. Your team will point it out to you.
Tag #5 - After you cross a bridge you enter a building will a lot of broken pillars. There are two Emergence Holes in this room. The tag is behind the second one in the grass.
Tag #6 - This one is in the large battle area with the fountain in the middle. The tag is in the back left corner if you face the logo.
Tag #7 - When you enter the outside area (in front of the House Of Sovereigns) with lots of stairs, there is a van with a logo on it and a big hole to the left. If you follow the edge of the hole up, the tag is in that corner.
Tag #8 - In the hallway where you fight all the wretches, the tag is in the left corner if you face the door Jack is ripping. It is easier to get it after killing everything.
Tag #9 - You will enter a small room where you find the body of Rojas (a dead Alpha soldier). The tag is in the direct right corner after coming down the stairs. Its behind the column.
Tag #10 - Immediately after the Berserker intermission sequence, the tag is in the flaming area to one of your sides. There is a logo in there.
Tag #11 - You must get the Berserker to smash down doors. In the third room before the final door, the tag is in the last alcove to your left if you are facing the door.
Tag #12 - Once you get outside after tag #11, you will enter a courtyard. Go to the left and follow the wall. The tag is behind one of the broken blocks along the wall. You must get this one before you kill the Berserker.
Tick Tick Boom
Tag #13 - After you split up but later meet again, you reach a room with stairs in the middle. They lead to a couch. The tag is under the couch.
Tag #14 - There is a room that you enter in which you walk on catwalks. After you leave this room, you go down some stairs that lead to the street. The tag is down under and behind the stairs.
Tag #15 - When you come up on the stranded gateway (not in it but prior), there is a tag back behind the newspaper dispensers. There is a logo above it.
Tag #16 - After the first checkpoint there is an outside area where an Emergence Hole appears. After that there is a shed to the left. There is a propane tank in it. Shoot the tank before going to get the tag in the shed.
Lethal Dusk-Dark Labyrinth
Tag #17 - This one is in the area that you must use the spotlight and guide Dom. It is in far left corner of the street, next to a pile of rubble. Shine the light on the tag after Dom has pulled the switch.
Tag #18 - After you exit a destroyed building, there is a tag to the right just around the corner. This is before you reach the gas station.
Tag #19 - After you split up to find an entrance to the factory, you will reach an area outside. There are little shelters along the way. There is a shelter with an elevator in it. Behind it is a dock that leads out to sea. The tag is on the end of the dock.
Tag #20 - It is in the room in which you must walk on the broken floorboards. It is in the far right corner. Make sure to stay on the metal beams to reach the tag.
Tag #21 - When you enter the room with the mine carts that you ride, go down the stairs and turn to the right. The tag is next to the stairs. Make sure to get it before activating the carts.
Darkest Before Dawn
Tag #22 - After you enter the caves on the drilling platforms, follow the path and stay to the right. Go down the right path and you will reach a steel door. The tag is in front of it.
Tag #23 - A little bit after tag #22, you will enter an area where you will fight some Locust, and there is a small building to the right. The tag is to the right of the stairs leading to it.
Tag #24 - After you and Dom split and meet, you will go down a narrow passage with Locust in it. After this you will see a Corpser moving to your left. There is a small ledge to your right. There is a tag on it. There is also some ammo at this location.
Tag #25 - When you enter the first area with a big fountain, look to the right. There are two small staircases. The tag is behind the second, or farther, one.
Bad To Worse
Tag #26 - After you leave Delta 2 behind, you will reach a street with a car to one side and a walkway with a ton of columns on the other side. The tag is behind the left side of the car (if you are facing the car abd not the columns).
Tag #27 - In the second section of the house you will enter an office-like room. It has lots of books and a desk. Destroy the desk. The tag is under it.
Tag #28 - This tag is just inside the door that Jack rips open. It is on the left side of the room.
Tag #29 - This tag is on the floor just before you climb the ladder to the roof of the train.
Tag #30 - Get to the second train car that you must drop the big tanks in order to cross. You should be on the right side of the car. There is a storage corridor to your left on the same car. You must go around to the left and into the storage room. The final tag is at the end of the corridor. Make sure to get the tag before you enter the next door.