To me, it doesn’t matter if it was left open for 5 minutes or all day. A CM opened the door (or worse, a guest with a mis-keyed MB (this has happened to me at hotel, very scary when someone unlocks your door at 1am)) and left it opened. The end. That is a security breach of the room, done by a CM. How long it was left open does not diminish the basic fact that the room was left open by a CM.
That said, the “someone should have noticed this open room and contacted the front desk” comment makes me laugh a little, sadly. There is such minimal housekeeping and regular staffing at the resorts, we saw almost no CM’s in the halls during our Christmas week stay. And to top that off, I don’t think any guest would even stop long enough to take notice of another person’s room door being ajar. They might think the guest left it that way intentionally if someone had to run out of the room for a minute, or that housekeeping was inside or security was inside. I just don’t think they would stand there and attempt to figure out what was going on and then report anything.
This falls 1000% on the resort management to resolve to the guest’s satisfaction, whatever that is. I would not be ok with just “we are sorry that happened,” if it were me.