His Bumper Sticker
William quickly realized how the officer actually knew that he had served. He had forgotten for a minute about the veteran bumper sticker that he had on the back of his car! Any person driving behind him would see this bumper sticker and would quickly learn that he had once been in the army.
A Symbol
It’s not uncommon for veterans to put identifying stickers like this on the back of their cars. It was very common for people to honk or wave at William in appreciation for the service he did, but he had never before been pulled over by an officer for such a thing.