How do spam programs find Adv guestbooks?

The question arose as we have recently installed an Advanced guestbook and found spam which seemed to be generated automatically. However, the GB hasn't even been indexed by google yet, so we are just wondering, how did they find us in the first place?! (I'm just asking for a general idea, not for a 'guide for aspiring hacker kids', as inquiring minds, we'd like to know...)

Robots, I think. If you do a search there is a discussion on this forum about robots. If you have site statistics available (usually offered in CPanel), I suggest you study them. Upon viewing my stats, I found all sorts of robots crawling my site - using up my bandwidth. Now I make sure to review stats on a regular basis. I have also learned how to disallow a lot of the bad bots in htaccess. Planning to set a robot trap at some point.