The graffiti (通渠 92263203 免棚) has been done by a plumber, Mr Yim Chiu-tong, who simply leaves his number and services on what must now be thousands of locations all over Hong Kong Island, Kowloon and the New Territories. You can be out on a hike somewhere fairly difficult to reach and just towards the end as you descend back into civilisation, there on the roadside is Mr Yim's recognisable calling card. This guy's handiwork is everywhere.
I recall Chris Dewolf wrote an article about him on his Urban Photo blog several years ago. Sadly the blog appears to have been removed but good old Mr Dewolf appears to be moving some of its content over to Zolima City Mag where he has written many great pieces on local history and culture. Coincidentally, he has redone/rewritten his article about Mr Yim and it can be seen here: https://zolimacitymag.com/meet-plumber-king-hong-kong-unexpected-graffiti-icon/.