Another option is if a false reg was temporarily applied for a film or similar, though I imagine it wouldn't have flown with it on. Saw a Gulfstream in an episode of NCIS the other night where they'd painted out the 'N', and I've an idea I've seen something similar in the past. Not much of a disguise, but I suppose since it was portraying the transport for a 'baddie' the owner may have wanted a degree of anonymity.