@Stephanie86
as they are adults, they can decide on their own, how they spend their days and nights 😉
I would send a message, that I hope they are enjoying themselves and just to remind them about checkout: when, how and that new guests are checking in the same day, so I need to clean the space and they really need to leave at ...
If you don't hear back from them till tomorrow morning, send them another message, that you need to know what's going on. You propose that you pack their things and take back the room, otherwise they would have to pay double fee for the following days, but this would be terrible for the next guests. (Even if you have none, you could need the space for your aunt Mary).
After that message, call airbnb, so they know in advance, what's happening. They will try to reach the guest too and have other ideas.