This has happened to me also, and ringing the guests didn't help as they had left the phone inside the house. They had forgotten to check out, thought they had another day. Anyway, there I was, new guests coming in a couple of hours, no idea what's going on and a mounting panic. I rang Airbnb, they said do NOT touch their stuff, rather clean around (maybe the bathroom or something, get it started while you hope guests will reappear). Airbnb offered to contact incoming guests and rehouse them, also to charge present guests. I said no, I wanted to honour new guests if I had a possible chance. So eventually as time was drawing to a close on the incoming I rang them to say I would probably be a little delayed, and I explained why. As I was on the phone to them the present guests finally arrived back at the house and were most surprised to see me. They were very apologetic, helped me clean the house, got out as new guests were coming in. Nobody charged, no locks changed. But Airbnb was very very certain that you can NOT touch belongings of guests.
Since you posted this yesterday, I realise it is too late to answer but maybe it will help someone with a future problem. Sometimes guests just make mistakes, not out of maliciousness.