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 ...