I wasnt able to lock any doors, with my own padlock.. Seriously... I can only think of one way that door suddently got locked, and I dislike saying it.
The only exploit involved abusing your intelligence
Game master could of locked it ;o
But really that seems like most likely scenario.
I checked those rooms to somewhere during this session. They were all open than.It's been locked for at least a few months.
No GM is doing such stuff. Doors lock themself after a time or when they close again.