Woo, my first post!
This is a tricky one...
If the patient room is private, and the patient has provided consent for the placement of the camera, then there is no HIPAA violation, though any videotaped footage can be "rescinded" by the patient at any time. If the room is semi-private, then yeah...major HIPAA issue. Also, if the camera faces the hallway/door, the potential for a HIPAA violation is also a risk.
The sign in your lobby is likely notification to the public that their images will be captured on security cameras in your hospital, such as near the exits or the OB unit. I would highly doubt it is intended to "cover" hidden cameras in patient rooms. Your facility would have to have explicit, signed permission from EACH patient to videotape in rooms. Managing this by turning cameras on and off as each patient dictates in order to "catch" a nurse in an inappropriate activity is HIGHLY unlikely. That would be a lot of resources to spend for little return...someone would have to constantly turn them on/off 24 hours a day...which is probably not likely to happen. In my experience, cameras are most often placed at nurses stations and ESPECIALLY medication/treatment rooms where narcotic diversion or supply pilfering could occur.
As for your second question regarding patient rounds, generally you will be OK if you are following hospital policy. In a court of law, this should hold up just fine, unless the patient's condition warranted more frequent observation, i.e. hourly, based on prudent nursing judgement. If most other experienced nurses would check a patient more frequently than policy dictates, then the court could hold you to that same expectation.
Never, EVER, try to use policy as an alternative to nursing judgment. If the patient's condition warrants 1-1 monitoring 24/7, then simply checking on them every 2 hours because "policy says so" could get you into trouble. Policies are usually intended to be the bare minimum...your expertise and good judgment should take precedent.
That being said, if the request for more frequent monitoring is made by the family, and the patient is stable (not a fall risk, etc), then two hours would probably be fine, though you could help maintain harmony on the unit by doing your best to accomodate them. If the request is made by your employer, well, then you're S.O.L.