Did you buy some new bikes this spring and summer?? If so, please take photos and document the serial number. This should be done for all bikes that you own. Most bike serial numbers can be seen when you flip the bike over on the bottom of the bracket where the two pedal cranks meet. Bikes are often made in bulk (lots of the same model and size).
Without the serial number police can not distinguish your bike from the next bike. So please document your serial number somewhere safe.