The Comet in S7E12 Up All Night
Ross takes The Gang up to the roof to see a comet "streaking by." They keep watching, but don't see anything happen.
I realize that Ross is a paleontologist, not an astronomer, but he's expecting the comet to act like a meteor. It's meteors that "streak by" (and there may be lots of them on the same night). Even though a comet is moving at great speed, it's so far away that it appears to stand still. So you don't sit and wait for a comet to show up, you look around and try to find it, which may be really easy, very difficult, or anywhere between.
When I was a kid, I saw a comet without even knowing it was there, simply because it looked like a comet and happened to be where I was absent-mindedly looking. But I couldn't find the famous Halley in 1986 until my uncle (an amateur astronomer) told me exactly where to look, because it was just a little fuzzy spot, nowhere near as spectacular as it sometimes is. (Oh well, it'll be back in 2061.)