Looking through the windows of a subject's parked motor vehicle has often proved useful to gain information (old envelopes, discarded wrappers/cups with logos, expired parking passes, notes, books and publications, and more).
Q: How do you eat an elephant?
A: One bite at a time.
Every little piece adds up.
Yes, this is another reason to use a UPS Store or other address for you registration and insurance card. You do not want your home address anywhere in the car in case it is stolen or rummaged through. As you said too, I often look through windows to see bills with the home address clearly displayed.