When my parents went to Las Vegas they stayed in the Luxor facing the airport. My dad took a picture of these unmarked Boeing 737-200’s remembering a show that was about Area 51 showing these aircraft going to Area 51. Sorry if the pictures are unclear but he uploaded them to the computer wrong.
Those are the JANET planes.
The new Flight Simulator X Deluxe has a mission in which you fly an early morning Janet flight from McCarran to Area 51. It even has a triangular-shaped “fast flyer” (Aurora?) that flies past you during the flight. When you land and taxi in, two of the large black mystery planes are parked on the ramp.
Sounds cool i’m gonna get my copy within the next few weeks.
Words of advice…
Don’t talk about flight sims on this forums… You will soon find out why… 
It’s okay! Flight Sim was mentioned as an off-topic subject and is therefore acceptable. 