I was at the Very Large Array (VLA) in New Mexico a couple years ago. It had a similar "turn off your phones and radios" sign and radio silence perimeter. They have tourists and a gift shop, so it's not as locked down as a military base.
I do also recall an episode of their TV show Click (technology based news show) that covered some form of enforcement that saw a patrol that would monitor and locate (via triangulation) and radio interference - be that leaky microwaves etc.
[EDIT ADD] I think I found the episode of Click in question, though it appear unavailable https://www.bbc.co.uk/programmes/b08450vk