Here is an interesting quote from, not existent already, Skunk works-mailing list from 1997:
"The other interesting thing about thursday morning flyers (as the guys at
Mugu and China lake call them) is the times they're flying- early Thursday
mornings, and their flight tracks.
1. Groom Lake operates such that flight test programs generally "prep" for
a test Mon-Wed/thurs, fly Wednesday or Thursday night, and spend the
remainder of the week going over the flight test results and doing all of
that fun debriefing stuff. Almost everybody has the weekends off to visit
family, etc. Groom is largely unmanned on weekends and this of course
explains the recent airspace changes around Groom for sundays.
2. The flight tracks for the sunday morning flyers point almost directly
into the Groom approach. Looks almost like a Space Shuttle coming in.
3. Recently the thursday morning flyers have started up again, though there
is no way to tell if this is the same aircraft or something new, but it
definitely suggests hypersonic/supersonic aircraft are being tested off the
California coast once again."
http://skunk-works-digest.netwrx1.org/v06-n076.txt
I'm reading it as Thursday morning flyers being what is commonly believed to be a LA "skyquakes" type... but then, the Sunday morning flyers? Never heard about that statement anywhere else.