Is this a problem

Noticed a while ago that the rear brake disc on my wife's Bonney was quite hot when she parked up...(don't ask why i checked/found this out because i havn't a clue). I decided to take the bike out myself on Sunday because I wanted to ascertain if it was a problem with the bike or my wife's riding style (backing it in like Rossi). Rode the bike for 15-20miles and didn't touch the back brake once........parked up and the rear disc was quite warm. I don't suspect the pistons in the caliper are sticking because 2-3 months ago I cleaned/changed pads. The only other thing I can think off is that the disc is warped.....advice/comments please.

Thanks in advance

Cleaning and changing the pads 2-3 months ago wouldn't mean the caliper pistons aren't sticking.

Triumh discs do seem to have a few problems, try spinning the wheel with pointer against the disc to see if it's warped, and check for wheel bearing play.
Oh and check the adjustment of the brake, pedal and actuating rod to the master cylinder.

put you bike on its main stand and spin the r/wheel, if theres drag on the disc?, look again at the calipers/pistons etc.
Is your disc discoloured or has shiny steps showing?. indicates a warped or Dished disc.
Also take your pads out and look for odd wear rates or uneven wear.
Also check for siezing brake pedal?.
let us know.

Or.............it could just be the pads yet to bed in properly. I once changed my VFR800 pads to EBC HH and they seemed to be binding for ages after. They weren't. they were just skimming the disc and hardly dragging at all, but you know what it's like when you've got something in your head.....nags worse than the..................

If its just warm then don't worry about it. The pads will tend to rub on the disc in any event as they are a loose fit inthe caliper. Its when you can use the disc as a cigarette lighter that you have to worry.