The Department of Justice has a fleet of planes for doing whatever they need to do with them.  I think they move detainees around but presumably their staff also can travel on them for business purposes.  Anyway, I might not know what the details are but I do know they are inconspcuous looking planes (unless you count a plane white airframe as looking more out of place than one with some color on it).  Two came in to Boeing Field while I was there a while back.  One was a 757 and the other was an ATR.  I was in place for the 757 as it was on final approach although the sun did annoyingly go behind a cloud as it came by.  It did depart alter but I was actually driving when that happened so no shots.  The ATR caught me out completely.  I had just pulled up after being away for a bit and was still sitting in the car switching the engine off as a white turboprop sailed by.  No chance to even get out of the vehicle let alone grab a shot.  That was actually the one I would have preferred but it was not to be.