Tunnel Above Shing Mun BBQ Area 9 [????- ]

Taking the path past the BBQ site up to PB 401, I noticed a cutting in the earth. Thinking it was just another trench, I hopped in only to find some kind of underground room. Didn't have a chance to explore, but the trench seemed well dug-out and of the size and shape similar to what one would expect leading up to a British PB; not the "hidden hole" style so favoured by the Japanese. May have something to do with the ruined block a bit further up the slope, or be related to the construction of the damsite.