Maybe you have to be a Doctor Who fan to be alert to any news with the words “Marco Polo” and “found” but that's how I just discovered a new app that I can't make my mind up about.
Get this new app, run it on your iPhone and the next time you lose your phone around the house, just shout the word “Marco”. Your iPhone will reply at top volume: “Polo!”
I have no idea why the maker chose those words. You can change the “Marco” bit but you won't because there's no changing the Polo response.