I would bet that this happens more often than you think. It's not that uncommon to see a lone bag or two on the tarmac.

I've had other pax take my gate-check item more than once. My ugly-green travelpro is easy to pick out on the luggage carousel, but can be an easy mix-up at gate check return if someone else has that same color and just grabs it without looking. Since you get their bag, you can probably track them down.

As for leaving the gate check tags--bad idea imo if you don't have the receipt.