Ohhhh, I thought you were referring to "Stealth Cosplay", rather than just a surprise costume that no one knows about yet or that is rather out of character for you. "Stealth Cosplay" is what I did for the mini-con: I dressed as Muriel Bouquet from Noir, but if you haven't seen the anime, you'd think it was regular clothing (since all the items can easily be purchased at the mall...a man's equivalent would ba Kaji from Evangelion because that costume also looks very much like regular clothing).
I've got a few other outfits that are fun to wear which are great to wear when on a trip to Japan (I did that in '02...and I actually had to stand in front of a poster of the character before someone noticed that I was cosplaying! ) To me, that's "secret cosplay", though I call it stealth cosplay becuase even anime fen don't always notice it.
see you all at the convention!