Rear brakes worn after 30,000 - caliper?

the sound you hear backing out is rust on the rotors. i would get a second opinion, and have them show you what they find if anything.
you might just need to have the slide pins lubed, which many mechanics miss, because they think everything is the same, and don't look up individual model instructions.
or, maybe the lug nuts were overtightened on that wheel. they don't torque them either, which can deform the wheel with an impact wrench.

 

Is he a "generic" mechanic, ie: it's not a Toyota speciality shop or Toyota dealership? Even the latter may not be well versed in one of the gotchas of 3rd gen Prius rear brakes:

When reinstalling the caliper, it's imperative that the caliper piston spoke pattern is oriented so that it "straddles" the pin on inner pad backing plate. Further, the pads need to be well seated (by brake pedal pushes), and perhaps a test drive, before any application of the parking brake, which will try to rotate the piston. A final test to ensure all is good: raise the rear of the car and try spinning the wheels (with parking brake off): they should turn fairly easily. Not completely friction-free, there's some overhead with disc brakes, but they should easily do one or two revolutions. IMHO a test drive and subsequent wheel spin test should always be done after working on the rear 3rd gen disc brakes.

If the wheels were dragging royally, it could well be due to not following the steps in the above paragraph. Also, the inside face of rotor would likely have the inner 50% looking rusty, due to lack of contact (due to the piston bearing mainly on the pin of the inner pad, not uniform contact). And the inner pad would have highly bevelled wear, toward the outer edge. The rotor's outer face might have a lot of scoring too.

The attachment has details on this:

 

Yeah I don't much like that there's special conditions you need to meet when reassembling the rear brakes: it's just problems waiting to happen. Poor design decision. Hope you get it sorted.

 

Well, it might still be a design decision that simplifies things compared to their earlier way of combining rear discs and parking brakes (as on the European Gen 2s), where you had a complete set of disc brakes and drum brakes on the rear....

-Chap

 

I kinda miss rear drums. They're maybe a step down in performance, but more reliable? Seem to last and last, with regular checkups.

 

I have that solution in RAV4. No problems. The drum is miniature for parking brake only, so it doesn't wear out (assuming no abuse). The disk part is standard construction and wears out 2 times slower that front. I grease the pins with OEM pink grease regularly BTW.