@Damon5
I'm sorry to hear that and I can immagine how bad you feel about it.
Maybe I am wrong but I don't think you can do anything now except tell ABB (in private , after you review your guest) that you suspect that guest has took your bag. It could be usefull if your guest do that again to another host and it ends in dispute.
If you had security camera and did cought him on tape leavig with your bag or if you didn't have other guest in the same time it would be different, I've heard of some cases when hosts were fully reimboursed for stolen items. But, maybe I am wrong, you can contact Airbnb and tell them what happend. Here is how to contact them: https://community.withairbnb.com/t5/Community-Help/Contact-Airbnb-A-Community-Help-Guide/m-p/16165/h...
Usually hosts who rent rooms in their homes have their bedrooms, valuables, money and documents locked for the same reason. Some of them have security cameras in their own bedrooms , it would be good idea if you do the same. We can not rely solely on good intentions of all our guests.