Bicycle detail

I rode my bike home and arrived at 6:30 PM on 9/16/2015. I carried my bike up to our second story apartment and left it outside of the door, next to my wife's bike. Now, I did not lock them, so you may just write this off as a case of ignorance, but please read on. My wife was making dinner in the kitchen, which has a large opening in the wall that faces the window (open at the time), outside which our bikes were parked. So, we would have our eyes on our bikes while cooking. At 7:20, our friends came over, and though they did not tell us until later (because they did not know I had a bike), they only saw my wife's bike parked outside our door. We ate together and chatted in the living room, right next to the window to our bikes, for the next few hours. At 10:30, one of my friends and I decided to go for a night bike ride, so we grabbed our lights and headed outside. I stared in disbelief at the empty spot which once contained our bikes. We drove and walked around for the next hour in vain. We asked our neighbors, but none of them witnessed the incident.