Doom49, a well-known Battlefield YouTuber has posted this video with locations of the dogtags.
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I found one in the very first room you can go into on the first mission, it's a knife stuck into the wall
gaming.stackexchange.com/users/58505/Schaeden
The AKE-971 is in TASHGAR, on top of the roof of a 3 story building just before you rescue some Marines defending a bridge.
gaming.stackexchange.com/users/15241/Ampersand
For the third mission, there is a dogtag in the room on the right just before the two guys whose fixing the light
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Stuff like that is hidden for a reason - to get you to explore your suroundings a bit without just rushing through.
I've played the campaign once so far, without necessarily trying to find every unlockable at the first try.
Ignore them for now, and enjoy the story (as generic as it might be).
The actual items are hidden in different ways.
Here's what I've found out so far (not really interested in looking up walkthroughs already): Weapon unlocks can be found in one of two ways: Dropped by killed enemies (just pick them up holding down R to pick them up; you can drop them immediately).
Just being there (again, pick them up and drop them if you want to).
Tis is especially true for the bigger anti-armor sidearms like missile and grenade launchers.
Dog Tags are hidden in one of two possible ways:
Granted for special actions, for example the final three dog tags are awarded based on your choice during the finale.
Granted for interacting with special objects (activated using E ).
Two examples (spoiler for first and last mission): You can pull a knife out of one of the school's boards and you'll find one collectible on one of sick bay's beds.
gaming.stackexchange.com/users/5486/Mario
There is a knife stuck in a wall on the way to the Cable Car.
Not sure which mission that was.
But basically you have to keep on the left hand side.
To get there you have to stay on top of the crates and buildings.
There is a jump you need to make to a walkway.
Then the knife is right in front of you.
It was just before a bunch of scaffolding if I remember correctly.
gaming.stackexchange.com/users/61380/Talon
I noticed in my battlelog, that you can find collectibles in the campaign.     Screenshot I took in the battlelog under the first mission  Where are these located in each mission?
Where are all of the collectibles located in Battlefield 4?
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gaming.stackexchange.com/questions/137930
gaming.stackexchange.com/users/18818/Zero
There's a knife stuck in the wall in the very first room you can go into on the way to the Cable car. The collectibles in Battlefield 4 are well hidden in spots. In order to find them all, a detailed guide is needed.
The community lists few locations.