Whatever Has Happened to This Fork?
I have seen similar deposited after stainless cutlery and plates were soaked in hard water from bores and from areas with hard water. Its just a collection of salts that like making a home on metal surfaces. You can remove it without scrubbing by adding some citric acid powder or a little white vinegar to some warm wash water. Denture cleaning tablets have a similar effect in cleaning off this buildup.

The citric acid powder is a really good way of cleaning up the built up scale in kettles and water heating appliances. It makes my glass electric kettle shine like new.