I had something similar happen to me a month ago.

I was checking in to a hotel at about 10:30 p.m. and they gave me a room on the third floor. I went up to the room, and it was propped open with the security latch. the room appeared to be dark, but I wasn't about to go in.

I went down and asked for another room, explaining what the problem was. While the desk clerk and I were both certain that the maid had left it like that, I wasn't about to walk in and risk finding someone in there. You were lucky.