Huh. You know I'd always interpreted that as her pointing out that there was nothing to read in his body. She's a human lie detector, they could have saved themselves a lot of trouble by pointing her at him from the start and just asking the question, he shouldn't even have had to say anything because his body would say it for him. So it must have been that Bruce was somehow physically giving nothing away, and I figured that that was what had Cass perturbed there.
Probably not what Bru meant by it though, no, now that you mention it.