[Outwest34au]

Hi FredZ

I am going with Chucks1 on this, my money is on the idea that the 'wrencher' used a product like hot wheels chrome wheel cleaner.
This product contains phosphoric acid 10-15% by weight.
Phosphoric acid anodises aluminium.

We came across this issue a while back, a customer bought said product and cleaned his Harley Davidson primary drive cover which was highly polished ally. It looked fantastic for a few minutes but soon dulled and went milky. After riding it the addition of heat made it go a faint gold bronze colour. This would polish out but the process would repeat itself.

It is possible to de-anodise the metal using a drain cleaner and water solution, a google search will bring up some vids showing how it's done.

Good luck.