The legal issue can be simply stated as follows:

If a person is in a location where there's a "Reasonable Expectation Of Privacy", then secretely making audio or video recordings is strictly illegal.

Don't bring cameras or other recording devices into a locker room or any other location or you might wind up getting charged with invasion of privacy.

privacy
 

Precisely - and instead of an inappropriate allusion to Harry cavorting in Las Vegas we should take account of Kate when topless with Wills on the terrace of their house during their private holiday in the south of France.
The French Court rightly upheld the subsequent complaint against the paps, out again
with their telephoto lenses needless to say.