Problem with Keychain

Hello,

I was reviewing the sites on my keychain a few days back and realized that there was a site on the keychain (username and password) that I had never visited and did not have a username and password for - any thoughts on how this could occur? Would keychain somehow pull in ad sites or perhaps this was some sort of virus? I'm a bit unclear as to how this would happen in accordance with my understanding of how Keychain works. Any help would be much appreciated.

Thanks