Paul Barford has drawn my attention to concerns that a Roman Julio-Claudian female portrait head sold at Christie's (London) this April for £91,250 (lot 261) surfaced with an interesting collecting history (a London-based blogger commented, "a head was sold at auction, the auction house said it had proof the head had been in Switzerland, and ... oh whoops, someone helping with the database has been able to prove not only where it was looted from, but also when (long after 1970)"). The person in question has yet to state the alternative collecting history and thus demonstrate that there had indeed been "looting". The key issue is that collecting histories appear to be fabricated on a regular basis. This is why I argue for authenticated documentation.
I would refer readers to comments on two pieces from the same April sale: a fishplate, and a Corinthian olpe.