@Rebecca2620
Send a message on Airbnb saying you are concerned they are ok as checkout time was 10am and they are not responding to a knock on the door and you are concerned for their safety. Politely advise checkout time is 10am and your cleaners are waiting to enter to prepare the space for incoming guests. I would also send a text message and leave a voicemail. I wouldn't let on that you saw them arrive this morning at 7am looking tired.
Then contact Airbnb and advise you are concerned about the guest safety as this guest is not responding and checkout time is 10am? Ask them to contact the guest as you are concerned maybe there is a medical emergency.
If all else fails, you can contact the police and ask for a welfare check at the residence, as you are concerned since the occupants are not responding to attempts to reach them.
Be sure to hold on to the video footage of their arrival this morning and whenever they eventually check out for future use. Also, if you have a smart lock, keep screen shots of their use of the code to enter and when they finally leave.
Hopefully you have a late checkout fee in your additional House Rules and sent them a check-out message advising of checkout time (Airbnb also sends a message about check-out time). I wouldn't send the charge for the late checkout until they write their review or the 14day period to write a review is almost up (I'm talking minutes here).