There's the whole 'implied knowledge/consent' thing, (ie we all are aware these days shops have cameras, so do they need to make it obvious, it's implied we know its a condition of entry now) but in general the employee has to be made aware they are being monitored, this would normally be inside your contract somewhere or some ancillary rule that's common to all workers for a site or signage as a minimum. If there's a sign at the building/site entrance that indicates CCTV is in use, then that's pretty much it as advice to both workers or customers that they are under surveillance. If you got caught with your hand in the till and were not aware of CCTV on premises, good luck fighting it.
Most times that CCTV is not used is if it's not in the businesses favor, it's their system, their footage, they will generally only use it to prove their own case.