There are a total of 35 dog tag collectibles in Aliens Colonial Marines. Finding all of them will earn you the "Remember the Fallen" trophy / achievement. They will also give you some nice bonus XP to level up faster. By holding down the select-button you can see how many collectibles you have found in the current mission and your objective is shown as well. After finding a dog tag it will be saved automatically. You can quit the mission right away without losing your collectible progress. Once collected, these dog tags cannot be picked up again.
#1 (Mission 1) - After freeing your comrade Keys, in the room where you encounter the first big group of Aliens. It's on a crate in the corner of the room.
#2 (Mission 1) - Look on a desk to your left before entering the second level of the circular room that has a rotating machine in the middle.
#3 (Mission 1) - At the very end of the mission, in the control room. It's in the back of the room on a crate.
#4 (Mission 2) - Right after opening the door with the loader it's on a box on the left.
#5 (Mission 2) - Shortly after finding Bella you need to cut open a door. You will come to a room with some computer screens. There is a small corridor on the right side of the room and at the end of the corridor is the dog tag.
#6 (Mission 2) - After fighting through waves of enemies in a big room (objective: get to command deck through engineering) you will come to a big room. Go up the stairs and the dog tag will be on a computer terminal.
#7 (Mission 3) - About 5 minutes after the mission starts you will be in a blocked corridor which forces you to enter a big room with lots of enemies. There are two doors that lead you out of this room. Use the first one to get to the other side of the blocked corridor and there you will find the dog tag.
#8 (Mission 3) - You will have to jump down some broken stairs. This one will be right underneath your feet after dropping down for the second time.
#9 (Mission 3) - After using the elevator, look to your left. It's behind a big container.
#10 (Mission 4) - In the room where you receive orders from Cruz. It's on a table near the entry door.
#11 (Mission 4) - After entering the room where you have to place the five motion sensors, make a right and go inside a small room. The dog tag is in this small room.
#12 (Mission 4) - In the long corridor where you have to kill Aliens with the minigun is a small side room to your left. Inside this room is the dog tag on some boxes.
#13 (Mission 4) - After picking up the last tag, keep moving down the long corridor and kill Aliens. You will come to a new, smaller corridor. You can either go straight and drop down or go left and drop down. You have to go left. After jumping down, turn around and the dog tag will be on the ground in front of you.
#14 (Mission 5) - After espacing the sewers you will have to go outside. Once you are outside, go left and the dog tag will be on top of a container. There is a big broken metal part in front of the container. Use it to get up there.
#15 (Mission 5) - After defending Bella and Reid you will see the boss Alien smashing through some containers. It will then chase your two teammates. Follow the path of the mission and look out for a big steel door with two small red lights on both sides. The tag is on the ground, in front of the steel door.
#16 (Mission 5) - In the room where you reunite with Bella and Reid. Look under the stairs in the corner of the room.
#17 (Mission 6) - After finding the dead bodies in the beginning of mission 6 you will see a dropship with searchlights flying around. When the dropship leaves, look on the ground near the cliff.
#18 (Mission 6) - This one is not far away from the last dog tag. After fighting some Aliens in a wide open area look for a green crate on the left side of the area.
#19 (Mission 6) - In the room where you have to retrieve the enemy personell. It's on the computer in front of the window that O'Neal uses to get the personell manifest.
#20 (Mission 7) - After leaving the caves there is a big turret with a searchlight on a mountain. Make a right and search for the tag on a table (there is another item on the table, too!).
#21 (Mission 7) - When entering the labs you will get to see a cutscene. After the cutscene is over, go into the next room and up the stairs. When you are standing in front of the door that leads to the next room, turn around and you will see the dog tag on a computer terminal.
#22 (Mission 7) - When you get the objective to destory the 3 power cells, your teammate will be standing at the control panel of a door, entering a password. Open the door and go down the stairs. Once you reach the ground floor, look to your right immediately. The tag is inside a box and hard to see.
#23 (Mission 8) - In the outside area, after reaching the first checkpoint, keep going and after killing all soldiers and aliens look in the last garage. The garage to your right when following the main path. The tag is on a box in the garage, next to an item.
#24 (Mission 8) - This one is in the underground area, inside the circular room that has an artillery statue standing in the center of the room. Go up the stairs and you will find the tag behind this big statue.
#25 (Mission 8) - After going back outside, instead of going straight to the captured marine, go up the hill to your left. In the back of the area, behind a truck is a small crate. On the crate is the dog tag.
#26 (Mission 9) - After entering the building in the very beginning of the mission, go up the stairs. You will come to a buring lab. The tag is in this lab on a table.
#27 (Mission 9) - After surviving the big Alien ambush (in the room where you can set up a turret) go down the stairs into the tunnel before leaving the building. The dog tag is in the tunnel, on the ground.
#28 (Mission 9) - After defeating that big battering ram Alien you will go into a building. Go through the door on the right and you will see the dog tag on a long, green workbench.
#29 (Mission 9) - Immediately after picking up the last tag you will have to go outside. You will get the task to disconnect the dropship fuel lines. Look out for some boxes near a set of stairs in the left part of the area. On one of these boxes is the tag.
#30 (Mission 10) - After using the elevator, keep going until you see a white marquee. After going past the marquee you should see two dead guys in yellow overalls in the alien goo. Next to the second guy is a box and on the box is the tag.
#31 (Mission 10) - When you come to the area where you have to destory the second AA gun you should see a buring forklift and a bunch of items on the boxes next to it. The dog tag is on the ground near the boxes.
#32 (Mission 10) - Towards the end of the mission you will have to help the crashed dropship survivors fight off the remaining Xenos. Between the two wounded men is the dog tag on the ground.
#33 (Mission 11) - In the area where you fight the Queen. After you gain control of your character turn around. Look on the boxes.
#34 (Mission 11) - In the area where you fight the Queen. From the last dog tag, go right. It's also on a box.
#35 (Mission 11) - In the area where you fight the Queen. From the last dog tag, keep going around the room until you can't go any further. There is a dead body and some buring engine parts. The dog tag is on the ground, it might be a little hard to see.