I just noticed last night that one of my rear calipers doesn't release completely when I let off the hand brake.

Looks like it's time for some Eric Shea rebuilds....

I had the same problem, rebuilds did the trick.

Check that the offending handbrake cable is not sticking due to rust in the sleeve and/or adjusted to tightly (yes Allan, there is an adjustment) .. disconnet cable at caliper and see if caliper arm returns to normal.

Check venting clearance .....

and you can adjust the cable lengths if one is too long (thus not letting it return all the way)

Just took a look under the car. It was the left side that was sticking.

As for adjusting the cables, it would seem odd that they would suddenly be out of adjustment so far wouldn't it?

The good news is that I was able to tap the arm on the caliper back so it would release.

The bad news is that I found a lower valve cover leaking....

Most likely what I would call "carmelized" brake fluid on the piston.

Basically see it "all the time". Old brake fluid mixes with moisture in the air (from bad dust cover seals etc.) and collects around the top 1/3rd of the piston.

Pads are probably fairly worn to allow the arm to stick. Arm sticking can also be an arm seal but usually, when those go they tend to spring back (rust infusion) vs. stick. Rust can bond the seal to the arm causing the arm to spring back not stick...

Probably the piston.