When I was in Free Mode, I was in a helicopter, trying to blow someone up as he killed me before. I chased him up to the place you shoot the Russians in the MeTV building in the mission Dropping In. I was in a Buzzard and I didn't know where he is. I checked every part of the building's outside, even the inside (while risking my chopper) and couldn't find him.
Later, I tried to see where he was before, I went inside after going on one of the base jumps. I went through some of the interior, going down the elevator shaft and going through doors, then I pressed Triangle on my PS3 controller and my player grabbed onto a ladder leading to under the floor of the building. When I first saw this, my reaction was: WTF?
I don't know whether this works on Xbox 360 and PC though.