I'm surprised that the OP wasn't more concerned about this. In particular, what happened to his/her belongings (laptop? business papers that may have been confidential?) that were left in the room? Was either party concerned about their security when the OP walked in on the family?

Something also seems to have gone wrong with the OP's keycard continuing to work on the old room's lock. AFAIK when a new key card is issued--and especially when it's done as part of a check in--the previous guest's keycards should be automatically cancelled. The new guest can easily walk in on the previous occupant (unless the previous guest is in the room with the door dead bolted) but the old guest usually discovers the problem when the key card suddenly fails to work.