Using a good RF scanner will work but like you said its not going to help much if the phones are off. Not to trivialize the issue, but a security guard that can frisk visitors will mitigate that risk to a large extent. Have lockers for visitors to put in their bags/purses etc before they enter the secure area.
What is the purpose of not allowing mobiles by the way? Is it to prevent people from taking pics? If so then the above physcial security issues would help in mitigating the risk. If it is more for preventing visitors from making phone calls while in the secure area, you can also consider moving the very high risk processes (eg cryptographic key generation) into a Tempest room/ Faraday cage which blocks RF signals from going out/coming in. thats really expensive though!
OSCP CISSP CSSLP CISA