@Clara116, Unfortunately, I do not think there is a way to prove theft to airbnb any more. If something is missing and the guest denies taking it or even noticing it, it would be an unexplained disappearance, specifically excluded from cover.
I had pictures taken 2 hours before checkin and they were disregarded in a damage and lost items claim. The guest’s statement to the contrary of photograped evidence prevailed.
Over the ladt 10 months, I had a sex offender, keeping up his written messages for days after checkout, a racist, where airbnb took down several of his reviews for me and another host, a review stipulating the place was so bad, the guest could not spend the night here, whilst I could prove she was here at breakfast, and a guest causing extensive damage. All of them are still on the platform. In case of the racist, taking down both sides of all reviews has done him a favour. He had answered relatively mild reviews with wild racist insults, which would have been a clear warning. He also confirmed that of course he had searched my room and the other host’s affairs. It was our fault for not locking up. The warning is gone, he is free to book again.
Therefore I think, the best thing to do is writing a clear review wich cannot be constructed as libelous. Speaking about your experience, how you lived it, should be safe.