"I forgot I turned it off."
Threepio was never even close to forgetting any of those 6 million forms of communication he liked to boast of . . . so how could a droid "forget" that he turned off the comlink while wondering aloud what was happening to his new master?
I'm sure he turned it off to keep any nearby Death Star personnel from overhearing the rebels, but I think it's silly that he could forget he turned it off.