What's needed is a single use stick on lens cover for mobile camera phones which is basically a disk of non-adhesive material surrouned by a ring of adhesive which rips easily to show if someone has tried to remove it...
On entry the sticker is applied, and if it has been torn before leaving you're in trouble!
The use of such phones in industrial espionage will prove to be more of an issue than illicit photos down at the gym. It is a problem that needs to be fixed.
Another possibility would be a bluetooth signal that could be sent t the phone to switch off the camera function, requiring a unlock code for it to work again... although this would rely on the phone haveing bluetooth
Just a thought.
PJE