Oil-like stain when cooking with gas
I've seen a number of varying explanations of this phenomenon, but all agree that it's a normal effect to see on stainless steel cookware and is harmless (assuming you are cleaning your pan well).  

It is probably some combination of the minerals in your water, high heat, and oil or soap residue.  The harsher detergents and sometimes less effective rinse of a dishwasher might make the problem more prevalent than if you wash the pots by hand.   It may also be caused by oxidation of the metal itself and you can remove it by scouring or with a bit of vinegar or lemon juice.

You can pretty much ignore it, unless it bothers you for aesthetic reasons.