Guy flips up opponents cards (Conversation with Dealer)

[rbenuck4]

Playing at resorts in Indiana when the following situation comes up.

Heads up at the Turn. Player A puts out a bet. Player B folds. Player A flashes his cards to player C, then tosses them in the muck. Player D grabs the cards and flips them up. Dealer gives a little smile (him and player D know each other). I give a confused look, dealer looks at me and says "Do you have a question?" in a not so nice tone. Appropriate response?