Do you have any side position
lights near this window?
I had this problem and just before undoing the window I found that the position
lights had lost their seal, the silicone was loose around them and when removing the
lights there was water that the buthyl tape could not stop because it did not close the passage of the wires.
I properly filled the lights with buthyl, redone the gaskets around the lights and my problem disappeared.
Suggestion according to my experience ...
Good hikes.