I heard about this myth several months ago from some users on Xbox Live when I decided to play this on my 360 for a while, we proceeded to do a several-hour long search for this thing, all of us in vehicles mindlessly traversing the tunnels. We found nothing but I was always curious after that and just couldn't let it go, I *had* to know. I am leader to plenty of clans, guilds and squads in various games and so I gathered up a bunch of my minions so that we could carefully explore the subways and any other below-ground area we could find. We broke down segments of the subway and made sure I had at least two of my minions at any exit - making sure the area was blocked off, the rest of the team searched the area. When we had thoroughly searched the entire area, we moved onto the next. Having devoted approximately 24 hours per island, I can confidently say the ratman myth is just that, and nothing more - a myth. It doesn't exist.
We didn't record it when I did this, so I know my comment is purely anecdotal - I'd be more than happy to have my mates give me a hand in doing it all over again, recording it and posting it here for the rest of you if it means quelling this ridiculous hoax.