Like Dave W, I have some 'stainless' items that have slowly stained (the French have it right- inoxidable). Steel spoons pressed from sheet have a dark tinge, other stainless cutlery are not affected. Different grades of SS?
Also, aluminium pots that have been used for stocks/broths seem to darken. Obviously different chemistry is involved.
I shall try citric acid for removing tea stains. When I worked in a factory, not cleaning cups was a mark of honour, boiling water addressed any biological hazard. So perhaps Duncan has it right. But the female householder at my location is intolerant of this viewpoint.
Bill