Just my opinion but there is nothing to cause the brake pads to move away from the disk except the runout or wobble of the disk as the wheel rotates.
If the disk is properly made this runout will be almost zero causing the pads to move a very small amount away from the disk.
That will always result in some slight touching between the pads and the disk which will cause a slight scraping sound.
If your car has disk brakes it is doing the same thing.
With the tire on a car pretty much surrounded by the wheel enclosure plus with the tire noise on the road and wind noise this rubbing sound usually goes unnoticed.
On your bike, with the disk standing proud out there in the air the sound is easier to hear. That is, it is easier to hear if the engine is not running.
Just start your engine and like magic, the slight scraping sound will vanish.