Tunnel up at Mount Butler, behind the quarry (22.268991N, 114.20667E) [????- ]

Craig gave some more detail on this one:

There is one on near the top of Mt Butler as you walk up past the Quarry stairs near the H057 marker, near where it splits towards Sui Ma Shan . The main entrance is blocked off by a couple of wooden posts that can be clearly seen while on the trail. It connects to two other openings. One right next to the trail about 10m away but can't be seen from the trail and the other right through to the other of the hill (30m) side looking over the harbor.

The tunnel is clear inside, apart from the odd bits of rubbish left by other visitors. You can walk, but it's just about shoulder-width wide, and you have to bend your head to avoid bumping it. Here's a typical section:

If you walk straight through, then as Craig says you end up out the other side of the hill. It probably had a clear view of the harbour when it was dug, but the hillside is overgrown now. Here's the view back towards this entrance:

When you started at the boarded-up entrance, the other option was to turn left after a short distance. That passage runs straight to a dead-end, but has another passage leading off to the left of it. Here are a couple of shots of the roof at junction:

You can see the marks left by the chisels used to cut out the tunnel roof. It is quite carefully finished.

There are also a couple of round holes in the roof, that look as though they were made by drills. I wonder if drills were one of the excavation methods,  or if there were rods inserted into the holes - eg with hooks for lights?

If you turn left, you'll come to the other exit that Craig mentions.

Seen from the outside there's only a small hole showing. Then behind that it drops to floor-level. It looks like there were steps cut into the bottom of the wall. As Craig noted, it's hard to spot from the outside.

The larger tunnels under Jardines Lookout also had a side tunnel near the main entrance, leading off to a second entrance. It seems a good idea to have a second way out in case the first is blocked, so maybe that was part of their standard design.

Finally, we didn't see the entrance shown on the link to Google that T gives above, so there must be another tunnel up there somewhere that we missed.

Seems someone has "moved aside" the boards on the once boarded-up entrance.  You are now free to go in by the big entrance right by the HK trail.  Saw 3 rats in there but they decided to make an exit via the harbourside exit.