He will find out! He will spot the familiar nibble-marks on Thor's arm. Or he will muzzily express concern for Tony's well-being, and Tony will be all "No it's fine, I'm nomming on Thor." Tony will be suprised and delighted with Steve's possessiveness. He thinks he could have gotten away with the dipping if he'd bitten Thor before going on stage.

Well, you'll need something for NaNo 2011, right? :D