When we replaced the window panes on our 77 Scamp
13 it turns out that I bought the wrong thickness of plexiglass and it leaked - I should have noticed that the gasket was a bit loose but did not. But I was able to make it work by taking out the panes and padding around the edges of the window pane with Gorilla tape! I had to experiment to get the right number of layers, but was able to get it tight enough that it does not leak. That is, except for the one I missed that I am going to redo this week!!
So maybe that is what happened at a window pane change by a previous owner - the replacement was too thin and it leaked. Gorilla tape was easier than trying to find and buy the correct thickness of plexiglass since Randy had already cut the windows