DM: Act natural, don't panic!
"Of course! Well, that, and Mr. M needed some paperwork brought to him. That's, well, what I've got here!" You're not exactly a good liar, but AA seems to believe it. Why would Mr. M need to get into Puzzle Detective's bank account? You know the boss does some sketchy jobs, and you know he isn't a fan of the detective, but you're hoping he's not stealing from him or anything!
"What kind of paperwork?" Uh oh.