Military toilet building above Pok Fu Lam Reservoir [????- ]
[20234 update: As noted in the comments below, this building housed toilets. It was part of the cluster of buildings shown at https://gwulo.com/node/32769, and is marked as building 'C' on the map on that page.]
If you look carefully you can see it as you take the steps down from the service reservoir to the main reservoir. It's about one third of the way down the steps, on the left. It has the wavy outline along the top of the concrete that we have seen on other, similar bunkers.