Found this out about 10m ago. No reason to point out who as I consider it a lesson learned. I'm an old schooler who still has piles of stuff in their backyard, so they went shopping a bit. Looks like all I lost was a few stacks of cheese, some potions, a big mound of SWF/red Fs I didn't have much use for, and maybe some other misc crafting stuff so nothing huge, but still annoying. I didn't do a full inventory and I'm not planning on it, as it'll likely just make me angry. I asked them not to take the stuff, once I saw them say "look at all this free stuff!" but apparently that wasn't heard. And I know now that I could have kicked them out, but this was the first time I ever had a house guest, so didn't know there were commands.
Seems like it should give a knock prompt? Then again, this might be working as intended, in which case, it's just a heads up. I remember from the old houses you rarely let people in due to theft, but I let my guard down a bit. Leader beware!