No remains. Even dinosaurs left bones. I don't think all humans who slew dragons managed to dissolve all the remains in corrosive substances. That, or dragons combust upon death.
The majority of creatures that die leave nothing behind over a few thousand years. If you have a species with a very small population it is entirely possible, though very unlikely that the members of the species would die in area's that will not provide the environment for things like fossils to form. That said the medieval period is close enough to us that bones should not have disappeared entirely, but then again if we follow the tiny population line, it is possible that we haven't stumbled across the few bone remains that are out there.
Either that or the dragons were turned into holy relics / destroyed as Satan spawn after their deaths.
