Going alongside what @Sarah977 said, why did you not try to enlist the police to help you retrieve it. I am notorious for sitting my phone down and forgetting it, so I constantly use the find my iPhone. It is pretty accurate, and as you probably know you can have it ping (are noise) once you get close.
However, that aside, as far as who stole it, it's hard to say. I too am highly doubtful it was the host. Eventually, a pattern of "missing" items would either be reflected in reviews, or brought to the attention of airbnb through multiple reports. As for housekeeping, again a regular pattern of stealing would not go unnoticed, however it does happen. I worked in the hotel industry for years and was surprised by the number of staff (not just housekeepers) who stole.
As for other guest, id say that is a most likely scenario. Dependent upon the quality of the locks on the door, some can be rather easy to bypass sans key. As far as why it would be popping up down the street, I don't know. My guess would be they would just shut it off and store it, but obviously whoever stole it is not as privy to tracking apps.
If it's still traceable, call pd and have them go to that area and see if you can pinpoint it. If its always popping up in the same area, then you can likely figure it's a persons residence. As for Airbnb refunding you, unfortunately it's unlikely, as the host did provide reasonable protection.
Im sorry this happened to you, and I hope it works out.