Legal Question in Technology Law in California

I'm not sure it's legally required, but it wouldn't hurt; and it might help deter misbehavior.

The answer depends upon the type of facility where it is installed, and, more particularly, on the reasonable expectations of privacy of persons photographed by the camera. If it is in a private dining room of a restaurant which business executives hire for luncheons to entertain their top prospects, the camera is almost certainly going to violate their reasonable privacy expectations and would have to be disclosed, presumably by prominently-displayed signs.

At the other extreme, if the camera is in the lobby of a bank, it's pretty clear there is no expectation of privacy that leads to a duty to disclose the presence of the camera.

Most in-between situations probably don't require signs, especially if the camera is visible and the area is more or less public.

A case worth looking at is SimTel Communications v. National Broadcasting Company (1999) 71 Cal.App.4th 1066. This case involved a business meeting at a restaurant, and the case held that the camera did NOT require signs, because the area was fairly public and not all the people at the table were "insiders" with a degree of privacy expectation.

Two other possibilities: ask the vendor of the camera what their recent experience has been, and look at other businesses similar to yours to see if they post signs when they use surveillance cameras.