Does the company renting your house have a copy of the key? Could be staff or vendors. I’ve seen it happen in the past.
I recall about 6 years ago many homes managed by the same company were found broken into after the season was over. The houses all had the electronic locks and the police found a cleaner using her personal code to get in any house that company managed. She obviously was not the sharpest tool in the shed it happens doesn't it.
Definitely happens! Also, our maintenance techs would somehow go into a house for basic maintenance things like changing smoke alarm batteries, filters, etc., and find a cleaner and her whole family staying in someone’s home without the owner’s permission. They’d drive here from inland, and just set up camp in the house. This happens frequently, actually.
We actually lost a really great, understanding owner once over this very thing. The cleaner was staying in his house while the house was empty, but he had a video camera on the back deck and saw her and her kids hanging out in his hot tub. He said he probably would have allowed her to stay there if she’d just asked; however, he rightfully felt this was a violation. Techs have reported it, but nothing is ever done, because they can’t ever find enough cleaners.