I was under the impression that this was supposed to imply that, because the transponders were off, the pilot would likely be aware of _something_ being awry (i

I was under the impression that this was supposed to imply that, because the transponders were off, the pilot would likely be aware of _something_ being awry (if even just that the transponders were acting up). Thus, an informal, nonchalant response might suggest that the transponders were intentionally disabled, and that would feed a suspicion of foul-play.