If that happened to me, being the guest in the room another guest walked in on... eventually I'd be mad but I'm not shy, so temporarily hilarious!!
On a more important note, you may need to be vigilante now with the front desk and your nights/points... your first part of the stay may now get credited to 2020 if they in any way checked you out and then back in... time to start making friends with the mistake makers, ASAP.