site of Tai Koo Sanitarium
From Quarry Bay, there's a pleasant walk up Mount Parker Road to Quarry Gap. As you reach the crest of the gap, just in front and to your left is a large flat area. There are some picnic tables there now, but it always looked to me as though it had been cleared & levelled for some greater purpose in the past.
When I read Tony Banham's 'Not The Slightest Chance', I noticed that today's Quarry Gap was referred to as 'Sanitorium Gap' in the 1940's. So it seems likely there was a Sanitorium building here at some point. [Wikipedia defines the term as follows: A sanatorium (also sanitorium, sanitarium) is a medical facility for long-term illness, typically tuberculosis.]
Then I forgot about it until Geoff's comment today about the Mount Parker hillside:
Or it could be part of the old network of nullahs that were built by Taikoo dockyard to the feed the dams that had to built as part of the agreement between Swires and the Govt. Essentially as part of the sale of the land for Taikoo they had to build dams to supply water not only to the docks but to both Shau Kei Wan and Quarry Bay. There was also a cable car system on Mount Parker that took the ex pat families to the bungalows in the hills, that were used mainly in the summer to avoid the heat. Pictures of the nullahs and cable car system are in the link to Historic Pictures of China (if you can be bothered to trawl through about 200 photos!)
The Historic Pictures of China he mentions are described here. One of the photos gives a clear view of the cable car (click the thumbnail for a larger view), and down in the valley you can see a white patch which is the reflection of the reservoir. I think this photo was taken from the gap.
A more careful look through the Mapping Hong Kong shows the 'Tai-Koo Sanitarium' on plate 7-1, a tourist guide map from 1896. It also mentions the building in one of the guide's suggested sight-seeing routes:
PROGRAMME No. 7
Proceed by ricksha to Bay View [a building that stood near where Tin Hau MTR is today], as the road is too long and uninteresting up to this point; thence walk to Quarry Bay, where, after having passed the entrance to the Sugar Refinery, about one hundred yards further on, just before crossing a bridge, you will see on your right hand, the pathway up the hill [the pathway is today's Mount Parker Road]. Something under one hour should bring you to the top, which is only a gap about feet 1,000 [sic.] above sea level. Here are the Sanatorium buildings of the Taikooo Sugar Refinery, Messrs. Butterfield & Swire. [The walk continues down to Ty Tam reservoir, up to Wongnai Chung Road, then down to Happy Valley, where 'we can obtain a ricksha to convey us to town.']
That same map shows a dotted line that runs from near the 'Tai-koo Sugar works' up to the Sanitarium. I wonder if this was already the cable car in place?
Plate 2-12 in the same book is a wartime map, dated 1939-40, and overprinted with Japanese. It shows the 'TaiKoo Sanitarium' building, connected to the 'Sugar Refinery' by an 'Aerial Ropeway', so it was in use then.
Plate 2-13 is a 1949 map. It still shows a dotted line along the course of the ropeway, and the outline of the sanatorium building is there too. Neither are labelled though, and the Gap is now labelled 'Quarry Gap'. So, perhaps the building and the ropeway were damaged during the war, and were never repaired afterwards?
More information about the ropeway / cable car here.
If you can add any more information about the sanatorium please leave a comment below.

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Taikoo Sanitarium
I think this is a photo of the sanitarium building - click the thumbnail for a larger view.