There are actually recommendations on the OPC website that say there should be a sign, that the sign should say what the purpose of the camera is, that it's there under the authority of a certain act, and tell you who to call.
Guess what? Toronto has a phone number for the Toronto police. Calgary says to call 311. You could wait on hold for two hours calling 311, so that's not an effective way, plus only the downtown LRT stations have signs. You go out to the sticks and you're not going to have any warning signs at all.