This honestly happens way too often. I've personally entered occupied rooms twice last year myself and have had someone trying to get in my room once all within one year. I make sure I always use the lock and chain when I'm in my room but doesn't help in the event I'm not there and they walk off with my stuff.

I've notice this happens when I tend to check in early and my guess is they pre assign the room (perhaps through mobile check in or something) and end up giving me the pre assigned room.