Unknown app from App Store wants me to update it

I have three Macintoshes all using the same iTunes (App Store) account. One of them wants me to update an app that I have never heard of, which App Store claims that I have purchased. When I look at the app ("Wallcat") on this Macintosh, the associated button is "Update," while if I look at this app on other computers the associated button is "$1.99." I do not have this app installed on any computer, and I had never heard of it before this.

I am guessing that this is just an inscrutable bug in App Store, but I have a little nagging voice saying "Security Alert, Security Alert."

Thoughts?