This sign is in the "Alien" Cafe and with 2 unsuccessful attempts to try to find the actual Area 51, we headed on back to Las Vegas in hopes of trying again on another trip. I think it's no surprise that we were unable to locate anything that told us we were on the grounds of the Nevada Test Site.
The owner of the cafe told us not to totally believe the words on the warning sign and to be careful not to trespass or photograph anything that was posted.