As you can hear in the video below, John's description was pretty apt. We made a few laps of the parking lot, but I knew almost immediately what the issue was. My old 2006 Volkswagen GTI had the same issue on occasion — a propensity to get rocks stuck in the brakes, usually between the dust shield and the rotor. It makes the most god-awful noise if it makes it all the way to the caliper. Thankfully, that wasn't the case here.
Option one is taking it to a dealer, but I have a garage and tools and was fairly confident I knew what was wrong. If the problem was more serious, I could deal with it then. It was also the start of spring, so working in the garage or driveway no longer meant I would freeze.
All in, it took me maybe 25 minutes to grab my tools, lift the car, pull the wheel off, find the rock and return everything back to its proper state, and I was taking my sweet time with the admittedly terrible photos. The rock was wedged exactly where I thought it would be. A screwdriver and some brute force took care of the issue, and John got a nice new showpiece for his desk.