Keep in mind that many secure no-cell-phone zones, even those that host classified data are still relatively physically open. The personnel allowed inside them are strictly vetted and trained to be self-policing, but it's only the threat of discovery and harsh punishment stopping someone with the right badge/code from physically bringing in a phone. There generally aren't TSA-style checkpoints or patdowns. Happens accidentally all the time, especially in the winter with jackets.
This is misunderstanding the purpose of the restriction.
The main reason not to bring a phone into the room is that the phone could be compromised. If the person is compromised then a device isn't your problem, because they could view the documents and copy them on paper or just remember the contents to write down later.
In a corporate environment no-camera/no-phone policies are sometimes also used for DLP reasons, out of expediency. Oftentimes it is more profitable to hire less trustworthy people (read: cheap labor) and simply make it inconvenient to steal data. This usually works good enough when you're trying to protect widget designs and not human lives.
Most people can take more photos then they are capable to memorize.
Can't you have one or more x-ray tunnels or other scanners? They don't even need to be actively monitored, just treated like CCTV.
Receving a full body x-ray every day just for a week would exceed the yearly federal occupational dose for radiation workers. You would add an additional 26% lifetime chance of getting cancer doing this for a year.
The yearly limit for rad workers is 5000 mrem with most receiving none. Receiving any dose is usually a cause for concern at most facilities that handle radioactive materials. A full body x-ray would dose you with about 1000 mrem. For about every 10000 mrem you receive, you gain an additional 1% chance of lifetime cancer risk. There's a reason why you wear a lead apron when getting X-rays at the doctor's office and why the technician leaves the room.
Metal detectors would be a much more reasonable method. People that work at airports, courts, jails, some schools, and even some manufacturing facilities walk through metal detectors daily.
Great points. Do metal detectors provide imaging capabilities? Would want to confidently move beyond belt buckle false positives...