@Tori2 If a guest texts or phones you about a potentially contentious issue, make sure to move it to the platform in the future.
Just text back that you only communicate with guests via Airbnb messaging, and to look for a response from you there. Then reiterate what they said in the text in the message.
"Hi XX, So you texted me about a wallet you think you left behind, but we've searched high and low, the place has been thoroughly cleaned, and it's obviously not here. You must have dropped it somewhere after you left, maybe it got wedged in a vehicle seat. Hope whoever finds it returns it. And of course if it does surface in some odd place, I'll be sure to let you know right way."
I took a taxi once and realized I didn't have my wallet after he drove away. I called the taxi company but no driver had reported it. 3 weeks later a driver called me, said he'd just done a major cleaning of his cab, and found my wallet wedged down behind the seat.
Good luck in court, I still don't think he can prove anything or hold you responsible for him not checking he had his wallet before leaving.