Losing Water badly

[itchy]

Hey guys,

I've just complete a 200 mile journey and my water light and 'STOP' came on twice. I reckon I lost around 500ml each stop.

I've looked for leaks but can't really see anything.

There is no mayo under the oil cap.

The expansion tank does smell odd and I also noticed black powder residue under the expansion tank cap.

Is this a dreaded HGF?

Thanks,

Nick.