If you see an unfamiliar person riding a bicycle carrying a backpack through your neighborhood, it's possible he is a burglar looking for some easy pickings. We've heard from law enforcement that a lot of ex-cons out on parole can't get a driver's license...so they end up on bicycles.
They look for unattended open garages where they can quickly steal some tools or other belongings - things that can fit in a backpack. It's not beyond them to break into a home. Electronics and other valuables can fit in a backpack too.
Our local police has instructed us to call them anytime we see someone suspicious, like an unfamiliar person riding a bicycle through our neighborhood and carrying a backpack. They'll come check it out.
Now don't call 9-1-1 unless it's an emergency. Memorize the non-emergency telephone number for your local law enforcement agency. Each person in our family has it programmed in our cell phones and on speed dial.