Mystery Solved
On the weekend the South China Morning Post revealed the secret of the missing vegemite (which I can't link to because the SCMP is still living in the Dark Ages).
Apparently, Japanese foodies have discovered that the taste of Vegemite has the flavour profile of something like miso - it has the "fifth taste" -
umami, which is kind of the taste of kelp. It's a lot more complicated than that - read the Wikipedia article for a better description. Anyway, these Japanese foodies discovered that Vegemite is available in Hong Kong and Singapore (due to the large Australian expat population) and so when they come to Hong Kong on food tours they raid the supermarkets and buy as much vegemite as they can find!!!!