I would be much more concerned if it was front brake and not rear.
As it is - remove rear caliper and inspect visually. Inspect brake disc and pads visually.
Satisfy yourself that there's nothing damaged and consider yourself lucky.
Next time get some kind of tether, bungee, or string that attaches to lock and throttle.
Handy reminder.
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2008 WR250X
Gearing: 13t - 48t
Power Commander 5 / PC-V
Airbox Door Removed - Flapper glued - AIS removed
FmF Q4
Bridgestone Battlax BT-003rs