When I was a child, and my Nana was still alive, we visited her often. When we walked up to her door, her mailbox was to the left, and it has a sticker on it that said "Carrier Alert." After seeing it many times, I finally asked my mom what it was for. She explained that it was so that the mail carrier would notice if the mail hadn't been taken in from the day before, and would have someone check on Nana.
Well, the Carrier Alert program is still around and their website explains how it works:
If you or a loved one live alone, consider signing up for the Carrier Alert program, as an additional layer of protection.