Other than a pen or marker, what would cause a "pen mark" to appear on my laptop monitor screen?

The answer is NOT a pen. I have a mark on my monitor that kind of looks like a pen mark, about a quarter of an inch long. It resembles a large “apostrophe”. It’s not on the glass. It’s under the glass and it’s black. I never heard of anything like it. I DO know laptop monitors sometimes have problems with pixels. Anyone ever hear of something like this? SONY VAIO, Windows XP.