These dogtags are pretty easy to miss.. in this video you will see the exact location.

Ambassador Dominic Osoba is looking for evidence of his son, lost during a mission on Benning. Find evidence of what happened to his son and return to the ambassador.

On your second trip to the Citadel, travel to the Embassies and then immediately head left. Ambassador Osoba can be overheard trying to find information about his missing son. Speak to him and you will find out that his son's squadmates returned from Benning without him but will not talk about what happened,and you can offer to help.

If you didn't speak with the ambassador,picking up the Dog Tag during a Hackett-sanctioned operation on Benning will result in this mission's acquisition. It is alternatively labeled Benning: Dog Tags in the journal entry.