I am fed up with this kind of stories, which are a blatant attempt at merit fishing. When they are accompanied by evidence, like photos or screenshots, I can believe them, but not the kind of story the OP tells, unless it is accompanied by a signed message to back it up, as you say. In a Spanish forum that I visit, not signing a message to prove what he says could lead to a massive report and a ban, although in this one, which is so tough for certain things, this doesn't happen. This is with some exception, as the OP could claim that he has the bitcoins on Coinbase or some similar site and cannot sign a message.