Thank you, Kimberly, for suggesting a solution. I tried what you said, but those are different marks. I'm going to describe it better this time, as I have a feeling you'll know what it is.

These marks look 2 small straight lines intersecting, at the top left and right margins. If I slide the margin scroller up or down, these two lines move with it. The one on the right looks like a letter L, while the one on the left faces the opposite way.

I looked for other possibilities in the Options area, but I'm still stuck.

Bart