Detecting an employee cam is not really all that difficult if done from the booth. Due to the geometry of the projected image on the screen, keystone distortion gives a combination of projection angle and viewer angle. Modern digital projectors have keystone correction. Old film projectors simply had aperture plates. Here is the difference.
An aperture plate is inserted into the projector to mask the sides and top and bottom of the projection beam to fit the screen. It provided no keystone correction. If a monitor test grid were projected, it would have keystone distortion with the lines narrow at the top due to the above audience projection angle. This applied to all 35mm and 70mm film projection. In short throw theatres, some barrel distortion is also introduced.
In digital projection, keystone distortion can be adjusted out by setting up the projector with a test pattern to make the geometry correct even with off axis projection.
No consumer phone that I know of has keystone correction for off axis correction of a film projected onto a flat screen. This will reveal the camera location when compared to the original projected image.
Most modern films are Digital, especially blockbusters. This means in most cases the projector has been professionally aligned to the screen with Keystone correction. With this knowledge, any keystone distortion and barrel distortion would be from the angle and distance of the camera from the flat projection surface. Shots taken from above the audience are taken from the projection booth.
With watermarking, a stray dot, blip, extra few frames between scenes, or other subtle alterations can identify which movie screen showed which film at what time. From there forensics can identify the general location in the theatre the cam was deployed. It's easy enough to identify a booth recording from the keystone.