Some topics are locked by an administrator and cannot be unlocked by the OP after that.
I remember this discussion coming up on the Meta board, but I cannot find the original topic (not even with my own search engine LOL)
To answer your previous question, I don't track any of that.
Found this with ninjastic:
If it's your thread, try unlocking it. If it doesn't allow you then a mod or an admin locked it. If it isn't yours, then AFAIK there is no way to tell (without obviously asking the author to try and unlock it). Not sure whether that's intentional or just how SMF 1.x works.
So I guess you just can't know.