Bicycle detail

I came to a friend's house at Newton court Apt 2S 3rd floor at 9pm. Since I wanted to stay there for a really short time, I just parked it right outside the door of 2S (apt is on the third floor) Hence I did not lock it. I come outside the house at 10pm to find my bike stolen. There is an old bike parked right near the staircase of 2S at 10pm which wasn't there when I came at 9pm. Hence the person who stole my bike swapped their bike with mine. I locked this unknown bike in case it belongs to someone and have left my mail address on it.