I worked for a company that occasionally would service some of our hardware onsite. One customer was a company that did a lot of work for the military and they had "that site" that a few folks visited. Here was how that worked:
Nothing except your body and your clothes left the site, anything you brought stayed onsite (laptops that we brought onsite were left behind / effectively disposable, later you couldn't even bring those, they provided one). All that stuff belonged to the military / whomever you interacted with at the site.
No electronics, cameras, etc that were not previously improved were allowed and you were told you would not be leaving anytime soon if you had something "unexpected or unauthorized".
It was highly suggested that nothing was in your rental car other than your keys, the equipment you needed as they searched the car and the folks would take what they wished.
If you realized you had something you didn't want to in the car it was highly suggested you do not turn around if you are at all close to the location and to drive up and immediately tell them you dorked up and brought something. This was a fairly remote location so the probabbly knew you were coming before you saw the gate and the guards didn't like surprises.
Upon arrival you parked, were blindfolded and driven from the gate to the site, you never actually saw the outside of the site until you were in the building. You were never alone at anytime. Trips to the bathroom while at the site were monitored... in person by a guard with a rifle.
Now all that sounds ominous but everyone reported that the folks there were very professional (not friendly but professional).
The point of that whole story was that even a while ago someone said "any electronics" were a threat and decided that they had to go to extremes to limit their access. Still today I think that was the closest to a "sure" policy.