Rear brake pads rubbing / sticking intermittently

Eliminate the mechanical causes first. The rear piston should form an X with the aligning pin going between the top arms of the x, and make sure you press on the brake to set that before you move the wheel and potentially rotate the piston.
Also, the polished metal clips can have excess rust under them, which prevents the pads from coming off the rotor. Make sure you pop them out, clean out the channel that they sit in on the caliper mount, and replace them if they're badly corroded themselves.

 

^ This. And it's a very common gotcha. You need the piston face rotated so that the cross pattern is like an X, assembled thus, with the inside pad's backing plate pin in between the raised spokes of the cross, and firmly seated thus (pump brake pedal). Hold off applying the parking brake until this is accomplished, and take a test drive.

Then apply/release parking brake, raise rear of of vehicle, and try spinning wheels. They should spin relatively freely. If the piston has managed to rotate (with application of parking brake) the pin will likely be riding on the spoke, instead of between two of them. This will drag royally, cause bevelled pad wear, and inside face of rotor will have limitted contact with pad, will have rusty zone.

Wheel should spin thus:

 

Also maybe it worth to take a look at those slide pins. Maybe they're stuck and need some cleaning/grease?

I've had same problem with my rear brakes. I guess it's about rust but even after I cleaned all places (including slide pins) the problem rise again after a while.
So I went at my brother-in-law and he grind a bit of pads "ears" so now they can move a bit in place. Problem solved since then.

 

Not sure about that:

Was this communicated to you by your mechanic? Might have lost something in translation? Pads don't rust to speak of, rotors will though, especially if the pad is not contacting properly:

What will happen, if the aforementioned X-pattern of raised spokes on piston caliper is not correctly oriented (and solidly seated thus), is the pin on the back of the pad ends up riding on one of the raised spokes. This is not as it should be: instead of having uniform/level pressure on the pad, the piston's pushing just on this pin.

Evidence of this happening: the inner face of the rotor (caliper piston side) will be half shiny and half rusty, due to the grossly unequal/skewed pressure.

The cure:

Assemble brakes with piston rotated properly, the X-pattern thus:


(shaded areas are the raised spokes)

Depress brake pedal repeatedly to solidly seat everything. Reconnect 12 volt neg cable (it should be disconnected at the outset). Do NOT use the parking brake. Take car for short test drive. When you get back, apply/release parking brake several times, then raise rear of vehicle and do test spin of wheels. They should easily spin 2~3 revolutions after you let go.

Above pic is screen-grab from the attachment:

 

The slide pins are some of the most overdiscussed parts of Prius brake calipers, it seems for no other reason than that pretty much everybody has heard there are slide pins there, even if they've never looked at anything else about the calipers.

Sometimes it takes a post like burebista's in #4 to remind us that there are more to the calipers than slide pins, and there are several spots that can bind up, sometimes much more severely than those overdiscussed pins.

In particular, the 'ears' of the pads slide in the springy metal clips of the 'fitting kit', which fit into channels in the mounting bracket. The fitting-kit clips have a dry, Teflon-like coating when new, which doesn't last forever; then eventually they rust. Greasing them is not encouraged because that would just capture road grit there. The mounting bracket channels those clips fit in can corrode, which makes them narrower, and clamps down on the clips, which clamp down on the pad ears.

Grinding down the pad ears themselves, as burebista's brother-in-law did, can get things moving again, but isn't really getting at the source of the problem. For that, sometimes it's necessary to pop out those fitting clips and file the channels in the mounting bracket back to the right size. Then might as well replace the fitting kit clips with new ones; it's probably time, and they're cheap.

 

Surface rust is normal, and not a problem.
The one photo of the inside pad looks like it's wearing on the lower section.
You'll have to pull the wheel off to get a better photo.
Rear pads last a lot longer than the fronts. And it also depends on HOW you brake.
If you are easy on the brakes, they will last a long time, even if they are installed incorrectly.

 

Yeah sounds like misaligned spoke pattern. It got ME first time I did rear brakes on mine. You can verify by wheel spin test.