One of the regular visitors at the library where I work is The Girl Who Knocks. She seems bright, and has an interest in a wide range of topics, often asking us to request books from other libraries on odd subjects. However, she also has some quirks. The most obvious is her habit of loudly knocking on the counter when she arrives in order to get our attention, even when we are standing right there at the circulation computer next to the door. She also tends to speak rather loudly.
I don't think she is trying to be disruptive though, because she quickly adjusts her noise level when reminded she is in a library. Scuttlebutt around town is that her mother was (still is?) a drug addict. It's a possibility. Methamphetamine has been a plague in the region. Remember, kids, drugs are bad, and drugs are incredibly destructive to the unborn.
As a tangential note, meth wasn't ever a problem until prohibition laws and the escalating "war on drugs" drove relatively less-harmful drugs out of the market by driving up the cost. The market was also far less violent than was portrayed in popular media. My source is a former Californian drug dealer who was active during the 1970s and associated with several biker gangs and other drug organizations of the era. So while drugs are bad, government is worse.
That aside, it is possible that this rumor is just that - a rumor - and she may have some other odd mental condition. She's just not very good at reading social cues. It could be something on the lower end of the autism spectrum, for example. I'm no expert at diagnosing such things. At any rate, other than her rather obnoxious initial knock when she arrives, I don't mind her at all, and her constant requests for numerous books on diverse subjects are always welcome.
Previously:
Monster Boy and Horror Girl
The Mountain Man
The Deaf Ashtray
Library Urchins
An Odoriferous Library Intrusion and the sequel