Let's say an extinct alien civilization inhabited an earth-like planet a few million years ago. The had a number of satellites, orbiting the planet, all of which got blown up quite violently (dealer's choice of method to destroy a satellite). Their satellites were in the same orbit radii that we currently use, namely a few hundred km through 40,000 km.
Would millions of years be enough time for virtually all that debris to decay? There can be enough debris left that you could find traces of it if you knew what to look for, there just has to not be enough that it would register as satellite debris in a routine scan of the planet and the things in orbit.
I'm not picky about the method of destruction, so a potential alternate question is: is there a plausible military mechanism of destroying satellites that would have the side-effect of leaving nothing in orbit after several million years?
It looks like any space debris below a few thousand km will degrade within a few hundred years (source), but there is a lot of atmospheric interaction in those orbits.
It also looks like intact satellites above 6,000 km would stay in orbit for millions of years (source).
But what about the debris from destroyed satellites?