The following are the locations of the dog tags in order of when you find them:
#5 Corporal Ross Dixon. Once the dust clears from the train wreck in the train area, go through the hole the train made. On the platform to the right, between two metal boxes, is the dog tag.
#4 Corporal Arthur McDonnel. In the Command Center (where you use the 51204791 code in the computer) reach the side where the computer is located. If in the box, the dog tag is to the left of the computer, outside. Go to the outside corner of the box, where you can look over the electric stream. The dog tag is next to the box. You should see it shining.
#1 Private First Class Hans Grant. After defeating the Boss from the freight elevator, the dog tag is left behind, on the treadmill.
#12 Lieutenant Mat Shuggart. After grabbing the parka, go up the ladder next to where you came out. Up there you should see the dog tag before the bridge across to the corpse and the detonator.
#11 Second Lieutenant Chris Ditter. After coming in the Filtration Plant through the tool shed (you came in a by a ladder), make your way to the left. There should be a slope with a little bit of ledge. Climb across that ledge and make it to the platform across. Get up on the little bit higher platform. Under the platform, is the dog tag.
#9 Officer Cadet Ed Collate. After getting the dog tag receiver unit, duck under the platform next to the staircase. Slice the trip wire, then collect the goodies: A SPR4 magazine, shotgun shells, a flame thrower tank, and dog tag #9.
#6 Sergeant Robert Cameron. Near the wrecked train, where you first meet the watchdogs, use the dog tag receiver unit to find it.
#7 Sergeant David Murren. Once coming through the door of the train room, you should see a booth or something straight ahead. Slice the wooden crates to the left of it and you should see it to the right of the steel crates.
#13 Captain Kurt Gianini. Up the stairs, in the same room, there should be a vent in the area where you look down on the booth area. Once in, take the first right and go straight until you reach the end. Turn right and there they should be at the end.
#3 Private First Class Tom Taylor. In the room with the metal tag, same area as the other two above. You should find it by the metal tag.
#10 Officer Cadet Lance McLaughlin. After confronting Cindy, you should find this tag in the corner near her.
#2 Corporal Filel Sidalis. On the ledge outside, near Filel, there is the dog tag on the ledge. Use the dog tag receiver unit.
#14 Sergeant Roger Grigman. After defeating Roger, the dog tag should be just before the bridge. Also take his knife.
#15 Major Mike Madigan. After defeating Roger, go to where the wounded Madigan is located. Once the FMV sequence is over, his dog tags will be next to the elevator-like thing where he descended into the Facility.
#8 First Sergeant Ken Reschner. In the area with the ice camo car, the dog tag is in the corner of a little outcropping. You should now have 14 out of 15 dog tags.
Expert mode
Complete the game after finding all fifteen dog tags and save. Start another game with the yellow "Rear Of Compound" save file (00:00:00 time). The game will begin with extra ammunition (500 bullets, 50 shotgun shells, 30 grenade rounds, 1,000 flame units, and 25 Ultimate Launcher missiles) and extra health items (20 Recovery Unit A, 5 Recovery Unit B, 5 MTS Vaccine, 5 Booster Shot A and 3 Booster Shot B). However, enemies will be more difficult in this mode.
Secret mode
Complete the game after finding all fifteen dog tags and Roger's knife. After the credits, and the funny candy girl song, you can save the game. When you do save the game, the save file will be yellow and have a star next to it. When you load this saved game file, you will begin with 300 bullets, 50 shotgun shells, 99 grenade shells, 1000 flame thrower capacity, and 25 missiles. You will also start out with an AT-6 Grenade Launcher. This has a six chamber revolving unit that you can use. You also get 25 recovery unit type A, 5 type B, 5 MTS vaccines, 5 booster shot type A, and 3 booster shot type B. However, the enemies take double the hits to kill in this mode.