If you ask the guy outside the commander's office who gets to come in, he'll mention the cleaning crew. You must walk out of his sight and wait one hour before you can tell him you've come to clean the office (with IN<4 you can get him to believe this right away though). You can now go in, but the commander won't let you open the locker with the Tanker Fob. Either Sneak-Lockpick the locker, or use (a lot of) Super Stimpaks on the commander. There is an easier way, for which you must have got this quest first (and need good Speech): tell the guard you've come to fetch the Tanker Fob, and the commander will let you walk right in and take it. In any case you get 3500 xp for getting the Fob out of the room.