The instructions I got basically said to have nothing in the car except what you absolutely needed, and that anything in the vehicle could be taken / don't have anything in there that might make someone wonder about what you would do with it (I'm thinking it would have been a bad time for photography hobbies).
This was before smartphones were super prevalent so not taking your phone really wasn't a big deal. Had my car broken down near the facility I suspect that I would have had "help" pretty quickly ;)
For all I know it was unnecessary, guys at the gate didn't talk much, although I did see them "inspecting" cars.
I got instructions from my company and folks familiar with that customer, it's possible they were just extra cautious. This wasn't like visiting a frequently visited facility or anything where you can look up the rules or anything.
