I suspect one of three things happened:
1. It was left behind by the tech who replaced the wheel bearing. It would have had to have been wedged in between the parking brake shoes, otherwise it would have rattled inside the rotor and made a ton of noise. This is plausible, except the rod is clean (no brake dust, nothing) which makes me think it's unlikely
2. It was wedged in there during the inspection for some reason
3. It actually fell out from behind the brake rotor. If so, it's back to what this thing is/where did it come from
4. If I want to believe that it was done maliciously, it would have eventually un-wedged itself sometime in the future and made a ton of noise, making me think something was broken and then having to take it back into the shop
I hope it's not #4.