The Enigma of Heston Grange: The Sequel to The Cryptic Lines (Unabridged‪)‬

What enigma is concealed within the darkened halls of Heston Grange? And where, exactly, is it hidden?

With the thunder and lightning outside increasing in ferocity, Charles read and re-read the words. Like his name on the envelope, the words of the message within had also been typed - this communication contained no actual handwriting whatsoever. At length, Charles allowed his hand to fall to his lap, the message still held between his fingers. Had there been anyone else present in the room, the words would now have been visible to all.   

The message read: "We know your secret".