Usually they don't need to count. In general working hours are stipulated in the contract, so if they find someone in the office outside the hours they have stipulated in the contract the managers better have a good explanation.... Then, if they consider the evidence is enough there's an official investigation where they can search for emails/phone call logs/documents that attest that the employees were working or asked to work outside of "working hours" and a trial takes place. In my current workplace there are no such strict rules, but if management finds someone working noticeably more than 40h a week they'll want to know why that person needs more time than the rest to do his job. I guess that's as weird for non Europeans as working >40h a week is for them.