I believe what you are experiencing is simple heat tint. The heated area should be treated with a pickling and passivation paste after welding to remove the heat tint and re-establish the passive layer. Alternatively, you can grind the area, but please not that simple hand wire brushing is usually NOT sufficient to remove the heat tint ( a common misconception by cheapskate contractors trying to save a few pennies at the expense of quality!).
I agree that metal transfer can happen when the workpiece is in contact with carbon steel (chains, hooks, supports, etc.). This manifests itself as rusty scores and marks on the surface. Think what will happen if a carbon steel wire brush is used on the SS surface!
I agree, the break press added an iron contanination layer to the part at the time of bending , also after heat bending 304 and 316 it is a very common thing to drop them in a cold water bath, this stops the carbon precip. and restores the austenitic structure to the stainless, take them out of the water, power clean the break press area of contamination and sand blast the heat marks if they are a problem...