Many years ago, my wife and I arrived very late at a hotel in Guadalajara. After putting our things down we stepped out on the balcony to look down at the pool. I closed the balcony sliding glass door behind us and - you guessed it - it locked. We spent several minutes yanking on the door, a few more blaming each other for our predicament. Finally (and mind you, it was like 2:00 am) we started throwing US pennies at other windows and got some poor person up and asked them to call reception and send a rescuer. When we finally got inside I looked at the lock on the sliding glass door. It had been installed upside down - so that as the door closed the the latch slid under the bracket and then popped back up into its underside. All this is to say, it may not have been an intruder, or a guest mistakenly given the same room. Bad design/installation is always a possibility.