Booth chuckles. "Don't worry, Ms. Echolls. I can handle my partner quite well."

The emphasis on partner makes it very, very clear that he doesn't want to discuss anything about their possibly salacious nature. After all, there's no point in denying it - it just makes a person look guilty if they protest too much.

"Now, back to the subject at hand. How did you come by your stepmother's credit card numbers? You used them just days after she had died."