Learn English – What does it mean to “draw a line under something”

Claudiu's answer is good. The original source of the quote posted by OMG Ponies is actually from The Simpsons. It's from Treehouse of Horror V; one of the Halloween episodes specifically. In this episode, Homer's "urge to kill"—i.e. his violent tendencies—are tapered by the Television working and/or showing a TV show that makes him want to kill (the episode was a spoof on the Steven King book/film The Shining, in which the character Jack tries to murder his family). It is, of course, a very tongue in cheek saying. It essentially means that one is extremely bothered and/or annoyed by something, so much so that it makes you want to "kill" something. The author of the post definitely means you no harm. :)

The cat above is peering/looking/peeking around a corner or a bookcase, or whatever this non-transparent object is. She would like the rays of light from her target to reach her eyes along a curved line, going around the corner; but that isn't possible, so she has to move her head partly "around" the corner, or bend her body around the corner, to see what's going on. The cat in your story is doing the same around someone's legs. As Aedia says above, it is the same as peering from behind something, peering while you're mostly standing behind something.

Looking around an open space, like looking around a room, is different: then your head or body is rotating around an axis, so that you can look in several directions successively. There is no blocking object.

P.S. To everyone: I apologize for the cheap cute-cat trick, but a picture was really the easiest way to show what's going on, and this one was on the first page in Google Images, and, well, it is a cute cat.

P.P.S. This one is even more impressive:

P.P.P.S. Oh, I really couldn't resist this one, I'm sorry...