Why Is Pen?!

I’m at my grocery store’s deli, paying for a few items at the end of a long day.

Cashier: “Here’s your receipt.”

Me: *Pen in hand.* “Cool, where do I sign?”

Cashier: “Oh, you don’t need to sign.”

Me: “…Then, where’d I get this pen?”

Cashier: “Oh… did I hand it to you?”

Me: “I honestly have no clue…”

Cashier: “…”

Me: “…”

Cashier: “Well, alrighty then, you have a better day!”

Me: “You too!”