Yet another new noise!

It seems my car is doing a Herbie and is upset at the though of the Z's arrival.

I have a squeek that sounds like squeeky brakes but its a bit random and doesnt do it while braking. I'm guessing its the rear caliper sticking slightly as you can stop it with the handbrake.

How can I tell if the caliper is sticking? Both rear discs were the same temperature (could only just touch them) and the calipers themselves felt quite cool.