Reading that article i would have to agree with Matt, the security should have done a proper search of the tunnels, because the odds are there is someone living down there. Security measures on sewage plants arn't that high compared to other places i've worked, i mean who in the right mind would want to live on one, they stink .
In the article they mention that they cant prove its a ghost due to high EMF's
I myself have worked in the Woodhead tunnel (and quite a few others), it goes between the Woodhead pass and Dunford bridge and carries National Grids 400kV cables through it. Now i'm not certain how much in the EFM range these cables produce (or how much is needed to produce hallucinations) but im betting its a little bit more that a sewage works 415v pump motors.
This leads on to a question for any electircal wizzo's out there, what produces the highest emf ? Voltage or ampage ?
I've been around very high voltage stuff on and off for the last couple of years and all i know is the safe distance before you get electrocuted but nothing about the magnetic fields.