Your bus leaves in a matter of minutes and you've somehow managed to lose your iPhone. You don't have time to log on to your iCloud and ping it with the Find My iPhone system - you need a quick, smart solution.
Or failing that, a game of Marco Polo.
The Marco Polo app allows you to find your lost device by listening out for your cry of "Marco". With the app running in the background, even if your handset is locked or on silent mode, the app will return your call with a shout of "Polo" - helping you to pinpoint it behind the sofa/under your bed/on the counter by your keys where you knew you wouldn't forget it.
Ridiculously, the app can shout back in one of thirty recorded voices (there's no Brian Blessed, unfortunately), and there are options to change your call and response - opening up endless potential for pranks by changing your friend's app to reply to a common word with a filthy expletive.
Those of you with a tendency of losing your iPhone/Pad/Pod can download the app here for £0.79.