I would imagine the security protocols of freeports like Geneva or Singapore make it pretty hard to fake any theft; and artworks valuable enough to be stored in

I would imagine the security protocols of freeports like Geneva or Singapore make it pretty hard to fake any theft; and artworks valuable enough to be stored in places like that are usually insured, so you'd have the insurance investigator to deal with. Claiming it's in storage while it's actually not: that might work, especially if it's something small and you can smuggle in a copy.