In the UK at least, if you believe that crimes are occurring (e.g. stock discrepancies suggest employees are stealing) you can get permission from your local police to use hidden cameras.

With that prior police authorisation it's legal to record more or less anywhere without those recorded knowing.

The police knowing you were authorised means when Sandra finds a camera in the employee locker room and freaks out, the cops who attend can verify that yes, Sandra's manager's boss got that OK'd two weeks ago when six £700 graphics cards "vanished" versus no this is some camera planted by an unknown creep and if the cops figure out who it was they'll be arrested.

If the "unknown creep" turns out to be the boss, the existence of this authorisation mechanism means they have no excuse - if they'd asked "Hey, can I secretly film my employees to get evidence of theft?" the answer would be "First, go to a police station to get guidance" not "Sure, act exactly like a pervert and just hide a camera".

What law would you think is being broken here? I don't think they are talking about bathrooms, where one might argue there is a "reasonable expectation of privacy." If you are working for an employer on their premises I think they have a right to do monitoring including clandestine. Just like they can monitor anything you do on their computer

I know several jurisdictions where recording employees place of work is illegal.

Like you can record corridors and such, but you can't record him programming at his desk.

Not in the EU.