We know who owns these planes, but we don't know where the planes disappear off to or what they're used for.
I thought they flew to Area51? This was resolved many years ago. All the civilian and military workers that live off base are flown in on Janet Air.
Janet Air has been around for many, many years now.
The author of the above info also got it wrong. Area51, also known as Groom Lake isn't secret, the base has been around for and been known about also for many, many years.
What is secretive about the place is that nobody knows for sure what is being developed there. New technology for sure, probably new aircraft are being built and tested there, probably new missile systems.
The only real reason Area51 is on anyone's radar is that for years it has been rumored that aliens from elsewhere in the Universe or local Galaxy reside, are imprisoned or come onto the planet and leave the planet from there.
No proof to any of that, but imaginations will run wild when military secrecy is involved in developing top-secret technological projects.
Wouldn't it be cool if it were true, though?