He was recalling the time when he was looking through the lighted window at night at a figure moving from right to left. There is a big difference between seeing that person through clear glass and seeing it through frosted.
Stipp's memory was that it was through frosted, because he couldn't see who it was or even if it was male or female. Just a pale figure.
Sorry, I think given that everyone including the judge, Nel and Roux all saw the significance of this suggests that no one is going to write it off as oh well, he's seen it before. Roux even tried to "prove" that Stipp couldn't really see the window - and failed.
He is describing what he saw that night.
You know what? If it looks like a duck, sounds like a duck, walks like a duck....it is probably a duck.
But if what sorrell said is true, then the window might have been partially open which is why he only saw a figure (I remember this). I wonder what this window looks like and how it opens. Is it a crank window? Is it one that opens like a door? Is it a sliding glass window? From sorrell's description it sounds like it's a window that opens sideways. So it is indeed possible that it was partially open. There's nothing definitive there.