yeah, i know... but Palpatine surely has his finger on the pulse of the dark side... he has a connection with Vader so that tight i can't imagine him not tapping in (let alone another dark side force user not showing up on his Sith Lord radar).
Part of the teachings of the Jedi and Sith is a mental block against other Force users, if Vader had something he didn't want Sidious to know then he could easily hide it.