Same in Switzerland. If there is a specific reason for surveillance the employee must also be informed of upcoming surveillance and the consequences if something were to be found.
You also cannot monitor employees using cameras.
In any case, you have to make that absolutely clear to your employees. Any unanounced surveillance is a criminal offence here.
Over workers, pointing at workstations no.
However, MTIM proxies by bluecoat ... Apparently is okay.