The US military operates a bunch of Gulfstream jets for transportation of key individuals.  One afternoon, I was at BFI and there was a C-37 from the USAF sitting on the ramp.  It had the powder blue livery common across the USAF VIP fleet.  I was hoping to see it takeoff, but it was not moving while I was there.  A little while later, it was joined by a second example.  This one was a lot less conspicuous.  It had no livery and no obvious markings at all.  The USAF has been toning down its planes recently and this was definitely in that style.  The lack of a registration might have told you that it wasn’t a civilian Gulfstream but the bigger giveaway would be the array of antennae that were on the fuselage.  This clearly could communicate with more than the average bizjet.