a thought

If - as I, being a member of the Snape-is-innocent crowd, believe - Snape made the Unbreakable Vow to Dumbledore and that's why Dumbledore always trusted him (because, seriously, "he felt really, really bad about the Potters dying" isn't a very good reason) then someone had to cast the charm. If it were possible for one of the parties involved in the Vow to cast it then there'd have been no need for Snape to involve Bellatrix when he made the Vow to Narcissa. Therefore, someone besides Snape knows about it and it's possible that person is still alive. Assuming I'm right about the Vow, any thoughts on who it might be? Nobody jumps out at me as an obvious choice. Just guessing wildly, it could be Alberforth.