Back in the 80s and I think also the 90s, on (German) TV, sometimes it happened that during recorded shows (I think only in TV series, but might also have been in movies) in one of the corners of the image (as far as I remember, it was always one of the upper corners, typically the upper right) shortly a simple geometric figure (triangle, square, circle) showed up and immediately disappeared again (I guess the length was just one frame). Often shortly afterwards (less than a second, if I remember correctly) another symbol shortly appeared at the same position. Typically those shapes were white, although I seem to remember that also black ones occasionally appeared; I don't remember any other colour.
Note that I'm not speaking about the station logo, which would normally displayed unchanged during the whole show. Those geometric figures would only appear rarely during a show, and immediately disappear again.
Unfortunately I couldn't find any example on YouTube. I also don't remember having seen those symbols later.
My guess is that those symbols are somehow related to the production process, and were left in either by accident, or because the specific production process required them to remain.
Does anyone know more about it?