To the OP,
In short, There are ony really two ways the disk can get hot
Ambient heat soak from another source -
Very unlikly, you'd know if you had a hot diff or transmission tunnel.
Contact / friction - Somthing is contacting the disc or rubbing against the disk.
If you want to prove it - And you do this at your own risk!!!!!!!!
Pop out the rear pads and push the caliper / pistons back
Go for a short ride and use the front brake only!!!!! check the temp in the rear disk
If the disc is cold, (and id put money on it being cold) pop the pads back in, press the brake a few times to bed the pads, and do the same route, using front brake only again ...
If the disk is hot its the caliper/ pad,s / disc relationship there really isnt any other way the disc gets that hot.
Check your runout on the disc it could be warped !
As i said in i think post 3 or 4 had exactly the same in the wifes car. only cure for her was a new caliper.
Try Bendy's method first, check the disc runout, if not then it's service the caliper / pistons for
£30 odd If it works job done,
If not its a new / refurb caliper / disc there really is no other route to go