Castle: Swan eluded his cameraman to go on a clandestine errand, an errand so important, he missed a band meeting. He was acting erratically, went to the bank right beforehand. This all points to one thing.
Beckett: That's an interesting theory, but that is pure speculation.
Castle: Actually, it's not. Pure speculation would be Swan purchasing thermonuclear technology to have a scientist make him an amplifier that went all the way to 12. This, musicians buying drugs, that's just Monday morning crossword.