Hello @Alexandra357 What a light and lovely studio you have! I can see that special care has been taken and your positive reviews support that. While you may have been a traveler on Air BNB for a number of years, it appears that you are fairly new as a host, right?
Your security deposit is available for claims of damage by the guest. Unless there is extra expense related to the cleaning a dirty space such as professionally cleaning a rug, the claim for cleaning will not be reimbursed. That also goes for the guest getting away with breaking rules. Unless the guest admits there were extra people staying in your place, or you have proof such as pictures, Air BNB will not be able to reimburse you for the loss of income.
You will need to look up the proceedure for filing the claim and what direct proof of damage and proof of cost, receipts, you will need to provide.
Of course it is understandable that you would think all guests are as thoughtful as you are or as the guests you have had so far. Apparently not so. Like being a defensive driver on the road, you need to be a defensive host. Lock up all that you do not want the guest to have access to. Left alone in your home is often interpreted by guests who do not have a sense of boundary and they will get into your stuff. Also, you could install a camera at the entrance as long as you post that it is there in your listing description. This will allow you to have proof of unregistered people entering your apartment.
I hope you will not be discouraged by this negative experience. No business is always smooth sailing so how you ride the waves and learn from the negative experiences will increase your success and satisfaction. Hope this helps!