I doubt it was anything Area 51 related. They have a large highly restricted airspace over Nevada to fly around in. However, Apple Valley is even closer to Edwards Air Force Base which has a long history of flight test including things like Chuck Yeager's supersonic flight, and for that matter the Virgin Galactic spaceship:

https://en.wikipedia.org/wiki/Edwards_Air_Force_Base

Many interesting things like the B-2 and the space shuttle were also produced in Palmdale (to the west of Apple Valley). You often see interesting things flying around that area.

EDIT: Virgin Galactic has nothing directly to do with Edwards, but they test in that airspace.