Act 1 - Ashes
Chapter 1: 14 Years after E-Day
Tag 1: In the room you enter after Dom asks which direction you want to go. Walk straight forward and it's on the floor in front of the Crimson Omen.
Tag 2: Toward the back of the room where the locust cut through the door. It's around a corner behind some rubble/fencing.
Tag 3: After exiting the room where the locust cut through the door, go up the stairs on the left and turn left again. It's in the corner.
Chapter 2: Trial by Fire
Tag 4: After entering the building, you'll soon come across an open area with a few low walls. After a short skirmish with a few locust, you can find the Cog Tag here amongst the rubble.
Tag 5: After a skirmish outside, you'll cross a bridge and enter another building. A few locusts come out of some emergence holes in here. Before exiting the building, you'll see the tag to the left of the door.
Chapter 3: Fish in a Barrel
Tag 6: Near the circular fountain where you have to close four emergence holes in a row.
Chapter 4: Fork in the Road
Chapter 5: Knock Knock
Tag 7: As you leave the building after the slow walking section and emerge out onto the street, you'll see a pale yellow van on some stairs in the distance with a Crimson Omen on it. The tag is near the van.
Chapter 6: Hammer
Tag 8: After meeting Cole, you'll soon come across a corridor covered with wretch footprints. If you go left, you'll find the Cog tag behind a low block.
Tag 9: When you find Rojas' remains, turn right and the tag is hidden behind a wall near the door.
Chapter 7: Wrath
Chapter 8: China Shop
Tag 10: As soon as the chapter begins, turn left and go to where the soldier just got killed by the berserker. The tag is on the floor next to what's left of his body.
Tag 11: While the berserker is chasing you inside, turn left before the final door and you'll see a window with kryll in it. As you approach they'll fly away. The tag is on the floor under the window.
Tag 12: After luring the berserker outside, don't kill her right away. Instead, go to the left and walk almost as far as you can. The tag is behind a wall covered in plants.
Act 2 - Nightfall
Chapter 9: Tick Tick Boom
Tag 13: At the very end of the chapter, just before you start the chapter "Grist", the tag can be found down some stairs and hidden under a couch. Destroy the couch to pick it up. If you've beaten the game before and are simply loading previous chapters to find tags, then I recommend you load Grist and simply turn around and go downstairs as that's much faster than trekking through all of Tick Tick Boom to get this one.
Chapter 10: Grist
Tag 14: You'll soon pass through a room with a metal catwalk. The right hand side will fall away and wretches will attack from the left side. After exiting this room and going down the stairs, turn around and you'll find the tag behind a block of concrete beneath the stairs.
Tag 15: Before entering the Stranded outpost, look behind the newspaper stands to the left of the gate.
Chapter 11: Outpost
Chapter 12: Lethal Dusk
Tag 16: After you cross the river and the kryll kill the Stranded at Checkpoint One, you'll have a small skirmish with some locust after moving through a building. Once the locust are dead, look to the left of the emergence hole and you'll see a small structure with a propane tank inside. Shoot it to make some light, then enter the structure to find the tag.
Tag 17: When you guide Dom with the spotlight, wait for him to turn the street lights on and then aim the spotlight at the pile of rubble and cars back where Dom started. Once the spotlight is shining on the tag, you can get it safely.
Chapter 13: Dark Labyrinth
Tag 18: After making your way through the house and being attacked by wretches a couple of times, you'll see a burnt out car with a Crimson Omen on it when you exit. Turn right to find the tag by the wall.
Chapter 14: Powder Keg
Chapter 15: Burnt Rubber
Chapter 16: Last Stand
Act 3 - Belly of the Beast
Chapter 17: Downpour
Tag 19: After your first encounter with Lambent Wretches, before proceeding up the elevator, check behind the building and you'll see a long wooden pier. The cog tag is at the end of it.
Chapter 18: Evolution
Tag 20: When you get to the room with wooden floorboards that fall away, explore the room carefully to find a path to the far right corner.
Chapter 19: Coalition Cargo
Tag 21: Before getting into the mining carts, turn around and look on the floor to the left of the steps you just came down and you'll see the tag.
Chapter 20: Darkest Before Dawn
Tag 22: Shortly after the chapter begins, there's a split in the path. Take the right, explore a little and you should find a Crimson Omen on the wall with a cog tag just in front of it.
Tag 23: Just after you fall down the waterfall and have a skirmish with a few locust, you'll see a small building to the right. The tag is in the floor to the right of the steps leading up to the building.
Tag 24: Just before the fight with the Corpser at the very end of Darkest Before Dawn, keep to the right hand side and you'll find the tag behind a rocky outcrop just opposite where the Corpser first appears. If you're loading previous chapters to get your tags, simply load the next chapter, Angry Titan, and backtrack a little as it's much faster.
Chapter 21: Angry Titan
Chapter 22: Tip of the Iceberg
Act 4 - The Long Road Home
Chapter 23: Campus Grinder
Tag 25: After the first skirmish in this chapter, you can find the tag on the right hand side of the area near the doors (almost straight forwards from where you start the chapter).
Chapter 24: Bad to Worse
Tag 26: As soon as the chapter starts, head to the right. You'll see the tag on the floor near a wall.
Chapter 25: Hazing
Chapter 26: Close to Home
Chapter 27: Imaginary Place
Tag 27: When you enter what seems to be an office of some kind with four pillars in the room and a desk at one end, destroy the desk to find the tag beneath it. You can also get this on your way back later in the Entrenched chapter if you wish.
Chapter 28: Entrenched
Act 5 - Desperation
Chapter 29: Special Delivery
Chapter 30: Train Wreck
Tag 28: After your fight with the Berserker, enter the door that Jack ripped and you'll find the tag inside on the left.
Tag 29: After making your way through a few of the rail cars, Anya will tell you that there are chainguns on your roof. Look around for the tag before you exit the car.
Tag 30: The second time you push a button that releases a bunch of large cylindrical canisters, proceed forward but don't enter the next car. Instead, turn around and enter the door. Follow the corridor to the end to find the tag.
Chapter 31: Pale Horse
No tags (I only included this in the list for completeness' sake)
Congratulations! You should now have all thirty Cog tags. I hope this guide has helped you out. If you have any questions, feel free to email me. My email address is on my profile page.