First Filter
My personal suspicion is not through the perfect happiness clause. We just had a series of comics come out that further the stories in our world and when I hadn't heard from him for a while I assumed it was because he was brooding over that. Unless there's something really damn happy in those comics - which seems unlikely considering its our world - we can suspect something else.
From there... I don't know. His soul is put in by magic, so it can be removed magically too. We've done it on purpose before, and it can be stored in a magical jar, or simply released. The latter is going to be easier to deal with, but the former - a problem. If you or Ichabod have stumbled across anything in your reality that can remove souls that you think might have relevance here let me or Sam or Dawn Summers know. We're sort of coordinating research.
As I told Dyson above, if he's been turning people it may start showing up not necessarily as murders (although any murder with bite marks should be noted suspiciously) but as missing persons.