Hi Michelle,
You can report tis to AIRBNB services using their email service or phone system ( it will take 5 minutes to find the number on google but worth it ).
I can not give them to you as i am in france and it is differents contacts but AIRBNB has an insurance service ( that is works even if you have your own insurance ) and they paid for the keys and change of the door lock after one my guest lost ( or simply kept ) one my sets of keys but denied he had done so.
My experience is i contacted the host first and asked informations about the keys, and became more precise in my questions as we exchanged. when he kept deniying i then contacted airbnb with a copy of our exchanges that showed some clear unconsistence which was enough to convince airbnb that i told the truth.
Hope it helps,
Best,
Nicholas