I've just gone through my collection and, sure enough, there they are on all the Hong Kong bunker boats. Just never noticed them until viewing your pic.

This metal DG flag is pretty common for Hong Kong bunker tankers.

Cheers

Vlad

Unusual, metal, red flag with DG for dangerous goods on the foremast. Never noticed that before

Thanks for your comment and information, Rick! I've spotted this one while taking a ferry from Aberdeen to Lamma Island, in order to check the viability of the northwestern tip of Lamma Island as a ship spotting location...

Cheers

Vlad

A dapper little tanker, very nice catch Vlad.
Delivered on 13/09/2016 by Fujian Heng Sheng Shipbuilding, Fu'an, China (HS-0028)
1,170 g.t. and 1,784 dwt.
Regards, Rick