Maybe someone else would take that as an opening to reassure him that he trusts him. Not Bruce. He takes it at face value, and nods, satisfied. "I won't lose it, and I won't tell anyone," he promises, signifying that he's going to take him up on that, and keep it somewhere secret. It can join the place he keeps the modified samples of his own blood that he'd used to research the cloning.

Then he's thrown slightly off balance by the compliment and the gratitude, and Bruce's business like attitude dissolves into a characteristic wry answering smile. "Not lately, no. You just like having someone to do secret science for you." It's a mild joke, not an accusation.