As a matter of principle, Shodan didn't engage in paper mail if he could help it. Therefore, instead of a note there's a tape recorder left in the Circle with a sticky note attached. It says "For October"
Upon playing the tape, the Libertine's voice can be heard:
"Hi! You ran away on us the other day so fast, I completely forgot that I still didn't tell you what's what in Sacramento. The FBI gadgets, the Seers, our own Artifact... anyway, swing by my place at [ADDRESS] anytime, or if I'm not around call me at [NUMBER]. See you!"