Japanese tunnel on Kowloon Peak [????- ]

I saw this on Wednesday, while heading to the top of Kowloon Peak for the first time. We took the trail that runs straight up the hillside from Fei Ngo Shan Road, starting from the graveyard. (Switch to the OSM map to see the trail.) There's a sign at the start saying the  trail is dangerous, but though it is steep it is clear, and has steps.

The position of the marker is approximate, but the tunnel was on the right side of the trail, about three quarters of the way up.

This is the view looking downhill from the tunnel entrance. The flat, open area and rocks on the side of the trail are a good landmark - the tunnel is just a little bit higher up.

Here's the sunken area on the right of the trail that caught my eye.

A peep in shows some collapse at the entrance, then a tunnel heading in and down, but I didn't investigate on this outing to see how far it goes.