Unless you're hiring, marrying, and raising the dumbest people around, they're probably smart enough to not perform those activities in front of a video camera in any case.
Security cameras don't usually have indicator lights, so the safe assumption is always that the camera is recording.
Security cameras also don't always look like security cameras...but most (especially Nest devices) do. However, if I was trying to find out some kind of nefarious activity within my home, I wouldn't put up an obvious security camera, unless it was to redirect the potential perp toward somewhere that has the actual camera - hidden as a book, or something similar - ideally, it fits right in with its environment, and doesn't look at all out of placce...
Security cameras don't usually have indicator lights, so the safe assumption is always that the camera is recording.