Some people!
I heard a story just last night about a recent late season camping trip. The trailer was winterised, but with the mild weather it was safe to carry a 5 gallon water bottle. Essentially, the person was out for the afternoon and evening. When they got up the next morning to make their coffee the water jug was empty. They checked for a leak or spill and there was no sign of either! Someone had entered the trailer and stolen their water! Sure, maybe the door wasn’t even locked (I didn’t think to ask) and that would have been a deterrent to start, BUT, if someone is motivated to get in, they will. The point is - most thieves are opportunistic.