A painting in the dark, and one scrap of it to find. Sweep with your window, lock it in, see how close you were.
foxing, the brown spotting that blooms on old paper
Run out and your window locks in wherever it stands.
Smaller patches are harder to place and easier to miss.
Strips are wide or tall, mixed gives you both.
Your window already matches the patch. All you have to do is put it in the right place.