Yep. You've signed many contracts agreeing to that by the time you're allowed to access anything secure.

At a prior job, we used to joke about "TOP SECRET cupcakes": once you brought them into the SCIF, they could never be taken back out.

While Snowden, WikiLeaks, and others are demonstrating that every country may be a leaky ship information wise, the scale of the USG information security apparatus is absolutely amazing, especially if you've also gotten to see how some smaller countries do it.

And as a visitor you must hand over all you electronic devices (which are locked away and given back to you on exit) before going beyond reception.

Years ago I interviewed at hunting's (a List X arms manufacturer) and the interviews where done in a separate room adjacent to reception you never even went inside the plant.

You don't even need a source for that its common sense, if you want a source try to find the CIA/NSA/GCHQ employee handbook or whatever material they give to new users.

Its the same idea that malware operators test out mobile malware in faraday cages so that the malware can't jump devices.