VHS was pretty mediocre. Any watermarking inside the signal would have been noticeable or destroyed in transmission.
However it's true that the top invisible scanlines normally used for Teletext could have been used for this. My VCR did record them, I was surprised to be able to view Teletext pages at the time of recording, though they were full of distortion.
It would have been possible to filter that out though.
I think at the time VHS recorders were still mostly analog and would have been recognisable from their artifacts the same way a typewriter can be identified once found.
A VCR isn't like most other forms of visual recording media where it takes an image and records it in some other format to be converted back to an image later, it's actually a 1-to-1 analogue recording of the broadcast signal (minus any degradation from being an analogue copy). So by design it includes any tricks they used to include metadata in the normal broadcast signal.
The mechanics of how they worked are actually pretty interesting, I highly recommend Technology Connections' video on the subject [1], as well as the one on closed-captioning [2] (and his series on analogue TV in general [3])
However it's true that the top invisible scanlines normally used for Teletext could have been used for this. My VCR did record them, I was surprised to be able to view Teletext pages at the time of recording, though they were full of distortion.
It would have been possible to filter that out though.
I think at the time VHS recorders were still mostly analog and would have been recognisable from their artifacts the same way a typewriter can be identified once found.