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.