I know sometimes mods lock threads at their own discression, especially at boards where they are a local moderator. At least when s thread gets moved there's some forum notice being posted. But for locking a topic there's nothing to signify it was a moderator action.
I just think it would be a nice courtesy if mods would just add a post before locking a topic to justify why they felt it should be locked. It's good to put this info on the record for future reference. And also from what I understand, since the OP can unlock a topic locked by a mod, a post by the staff member that took the action would prevent misunderstandings.
I have had my post locked also for no clear reasons sometime back also, but it seemed like the mod understood I had gotten enough satisfactory response for the question I asked and did some good to lock the post, I guess.
I love the fact that this came up here and in my opinion, even if a justified post isn't created, at least, a dm or email with a good reason for locking the post should suffice as a courtesy to the person who put in the effort to create the post.