Just thought I would update this. The reason that we couldn't find the how the reporting server could be sending a non-existent subscription was that it wasn't. The report had originally been created on an older reporting server instance which is mostly unused now, but still running. The "phantom" subscription was coming from the old server. Easily dealt with, once discovered, but annoying.
So I am happy to report that there are no zombies or ghosts here, just old servers that get forgotten.