Things that go creak

The other day during the SAS training I attended. I forgot to mention about this strange incident that happened. It was during the tea break when me and my other colleagues were standing in front of a training room that was unoccupied (lights off and all). The door suddenly creaked open by itself! Not fully open, just ajar. Dg noticed the sound of creaking and the motion of the door gently open and looked back at me. Quickly Dg closed the door and we scurried back into our training room. Bizarre!