How very inconsiderate!!!
I would suggest putting their stuff to one corner of the property and get on with cleaning the property for your next guest. Send theses guests a message informing them that they are way past their check-out time which was at noon, you have attempted to make contact them but they have failed to respond and also failed to communicate effectively prior to check-out by enquiring if it you could accommodate a late check-out.
Inform them that you have a guest due and that you are contacting Airbnb regarding the matter as well. In addition, inform them that they must confirm what time they intend to collect their belongings as the property has now been locked and they will be unable to gain access, that on arrival, they will only be permitted to collect their belongs but not to access the property.
It is vital that all your communication regarding this matter is done via the Airbnb messaging platform, text messages, WhatsApp, personal mail etc. is not deemed acceptable evidence to Airbnb. As to whether you can charge them an additional day’s fee/ late check out will depend on (a) what time they eventually return (b) contents of your listing , is a fee for late check-out mentioned
link - Contact Airbnb
I hope you get hold of them soon!
Ama