My mom always had this thing about not flushing the toilet after she pees. Ever since I was a kid, I remember lifting the toilet lid and seeing a small mountain of toilet paper just floating away. Although it's not a crisis like David Sedaris' poop story in Me Talk Pretty One Day, it's still a rather annoying discovery.
Nobody wants to work twice at flushing. It's a very perilous act depending on the toilet. You could be sprayed or be the lucky person who gets the clog. And really, how proud can you be of your waste that you need to save it for the next time.
I don't know what she thinks she saving by leaving the flush for the next person. Saving water? Hydraulic energy? Maybe she's looking for praise or leaving it as a reminder that indeed she did relieve herself because she's always forgetting her keys, so why wouldn't it slip her mind that she tinkled? And perhaps she just plum forgot to flush but then why bother close the lid? Just forget it altogether - Pee and Flee - it's a little more understandable.
In case you're wondering, I did bring it up a long time ago when I lived at home. Her response was typical of any confrontational issue, "Well, you got a lot of nerve."
How asking someone to flush the toilet after they use it equates as having a lot of nerve, I can't say but I think the answer is to hide all the toilet paper from her the next time she comes over.