OK that thread was not locked when I looked last night and unlocking it and locking and unlocking it again shows in my log of actions on the site. And when you lock a thread it doesn't give a warning you are doing it .
And the important thing is you as the thread creator CAN remove and lock it. And if you aren't used to having this options there you can easily hit them thinking it is reply (those icons should be on the left of the page whereas reply is on the right).And it will tell me in the logs if a member removes their own thread but I don't think it does when they lock it.
Short version I think you are the one locking it by accident.