Up until now I've been searching every floor of every building I go into for a mission. On most floors the elevator doors won't even open, but there are always a few floors that have small areas you can search around in (in addition to the floor where your mission takes place). I think the only interesting thing I've found on any of these floors so far was when one of them had a door that opened up to a brick wall directly behind it (which I found amusing since it reminded me of the deja vu incident in the original Matrix film). I've never found any items hidden on these other floors, however.
As my missions are starting to include buildings with 18+ floors, I have to ask.. has anyone ever found any reason to check the other floors in a mission building? Or am I wasting my time on this?