Free Starbucks Drink Mystery!

Hey everyone!
When I got the Iced Caramel Macchiato yesterday, I knew I had less than $3 but for some reason the cashier didn't ask for any extra money, so I assumed I had like $3.50 so just enough for the drink. I came home and checked my Starbucks Rewards account and saw that I got a star (which you get for each purchase) and the amount was around $2.40. I have no idea where that extra $2 came from but I'm not complaining. Then I checked the transaction history and all it had was until May 20th, and no more purchases after that.

Those were all checked on my iPod, so today I decided to check online and it said that those were all of the transactions until May 24th. Then I realized that yesterday, the barista gave me that Caramel Macchiato for free! Right after all of that complaining of how expensive it was. Isn't that awesome? Anyway, I decided to share this with you. I'm thinking of going back to the store and telling them, because I don't feel comfortable otherwise. Or should I call the store and tell them?

What would you do?
Bellesme