At the grocery store, an employee came up to me smiling and said, "you shop here a lot." This has never happened before; I had no idea how to react.

When your in a small town it rarely gets asked. No have no options! 😞

Could have been an ice breaker for a pickup line

Not that unusual. I worked at a grocery store for 12 years. Some people came in daily, some weekly, some every 2 weeks.
I live in a huge city of millions and yet I was used to seeing many of the same people. Some were quiet and we never struck up a conversation yet I still remember many of their faces years later
That random guy who saw you might be extroverted and just said something to you, no biggie.