Unless there is a law against it, the employer can put cameras anyplace he wants. Bathrooms and locker rooms are probably off limits but I've not checked.
Likewise unless there is a law that says the issue must be posted, he need not post--but I suspect the employer is better off to place modest signs at each entryway that says in effect these premises are under survailance (sp). I suspect there a defensive need for such signage--note you'll see it in many stores as well.
Unless there is an expeactancy of privacy as in a bathroom I fail to se any limits on the use of the recordings.
If you are on camera making it with your friend on the picnic bench on my premises I suspect there are a lot of things I can do with that, provided I stay clear of extortion ! And if you appear to be engaged in a swap of money for a small packet I may be able to invite police to come help me watch the replay.