I agree with Matthew, this too is why we got an electronic lock with a keypad. I'd highly suggest looking into getting one so you can avoid situations like this moving forward and make the overall space you're renting out that's available as easy and convenient as possible for both you, and your guests. (:
We just got this new lock a couple months back and absolutely love it! It syncs directly to my AirBNB calendar, sends guests emails with key code and instructions as soon as they book a stay, enables codes at time of check in and disables codes at time of check out (and accurately lol), and auto updates itself for me. I don't have to do a thing or have to worry about keys or anything myself, it's awesome.
Here's a link to check it out, I highly recommend it!
https://www.amazon.com/AirLocking-Smart-Lock-for-AirBNB/dp/B0759NPC5Y/ref=as_sl_pc_tf_til?tag=airloc...