Hello, am new in Airbnb. The only thing I would think you can do is change your lockpad on the door, and for future situations, make yourself a copy, and attach to the key a huge red ribbon, with multiple knotts, that is what I do. I have rented my cottage in the past by myself, and had some guests to carelessly mishandled the key and consequently lost it. Before they leave, I ask for the key, and I do a revision of the property to see if they stole or damaged things, I have nice oil paintings, and is always a concern for me. I ask them for the key on my hand before they leave.
I have learned through my experience ,as a host, that guests pick up on your cues in how you carry yourself and how organized and precise you are. I try to give a very professional impression, on the way I dress, the way I explain to them about the cottage and the town. I also do not bend and accommodate on ridiculous requests some have, I let my reasonable standards speak for itself. So I tell them--there is a fee if one loses the key, so please mind the key. What else can you do really? And I don't see how Airbnb can do much in this situation. Somethings are out of their hands. If you do not have another key, you need obviously to change the lock. I hope my comment helps you. Good luck!