Or a galvanized steel water storage tank? Galvanized steel hot water
pipes?

The bee poo theory was promising.

I'd be looking at the water supply and storage inside the house. Is
the cold water supply from the mains? Is the hot supply from a combi
or a storage cylinder?

I once heard of a problem with a leak from a district/council heating
system. It eventually turned out the family wearing brown clothes had
plumbed their washing machine in themselves ;-).