Picked up my winter tires this morning, and the shop owner put Miss Peanut up on the hoist. Mr. Brig got right in there. The owner was very polite and accommodating. As it turns out, my wheels hadn't been torqued to 100 ft. lbs. Technician had only done 80. (They document all jobs via the system, as well as via CAMERA footage!) Also, this shop, as a common practice, puts some kind of lube around the centre of the rotor (or maybe the centre of the rim) to preserve alloy rims and/or prevent seizing? Not sure I fully understood. In any case, the tech had WAY overlubed the rotor, and Mr. Brig thinks that because physics, this combination had the effect of loosening the nuts over time. He says he's never seen that done before, and doesn't think it's really necessary.
He asked the owner to clean up the rims and get rid of all the lube, which was done. I was fully reimbursed for storage that I wasn't going to use (obviously), but also for the tire swap, for a total of $144.
Case closed (I hope).