In supply chain portion is usually kept to a minimum in normal...

In supply chain portion is usually kept to a minimum in normal conditions. Plenty of zinc smelters within trucking distance to customers which can have very low stock in their supply chain. Lets assume 50% of the 940k hidden stock is ‘supply chain’ stock. Still plenty of zinc in off-market stock.

There is a reason for the traders/smelters to keep stocks hidden - so it isn’t common knowledge in the market And expensive/hard to obtain this info. If it was common knowledge and easy to track it would be much easier to find this info.

Temporary high zinc price = most recent spike to around US1.20/lb in the last month.
18 months ago the stockpile of zinc in metal form did not exist and concentrates TCs were much lower so a very different environment.

Look at the zinc metal premiums and TCs now - It is a very very weak market with no end in sight.