Hello and first of all welcome to my guide, I will definitely help you how to get to 100% discovery.
There are pictures to see how you can do it.
The red-marked color are for Hidden Roads.
Western District
There are only 2 Hidden Roads far.

Central District
There are 0 Hidden roads far.

Wan Chai
There are 0 Hidden Roads far.

Causeway Bay
There is 1 Hidden Road far.

Quarry Bay
There is 1 Hidden road far.

Chai Wan
There are 2 Hidden Roads far.

Shek O
There are 28 Hidden Roads far.

Tai Tam
There are 24 Hidden Roads far.

Stanley
There are 8 Hidden Roads far.

Happy Valley
There are 10 Hidden Roads far.

Aberdeen
There are 17 Hidden roads far.

Victoria Peak
There are 9 Hidden roads far.

Pok Fu Lam
There are 6 Hidden roads far.

Repulse Bay
There are 4 Hidden roads far.

Done all that and still stuck at 99%