Someone put passcode on iphone 4S

I accidentally left my unlocked phone lying in a store and someone put a passcode on it and i don't know what it is! Please help! I have tried many times, but now my phone is locked for 15 mins.

Connect it to ITunes and there's a to unlock it from there. I had a similar problem with my iPod, but I started at 0001 and guessed at the code, joint the # by 1 each time until I found the right code.