salt water leaking under the crystal. I can also see more pits on the rehaut just under the crystal. Have you had the watch from new? If not, the previous owner may have had this problem which has slowly crept up over time.
Salt water and any fluids which consist of halides attack stainless steel and most less noble metals in general. Also free iron particles do the same. I have addressed this topic numerous times in the past, as I believe it to be paramount for vintage collectors which is a subject that often gets overlooked. So much fuss over original dials, types of bezels, over polished cases etc etc, but the most important element of the watch in my opinion is the condition of the case which what protects the watch. Once pitting begins you cannot really stop it as it feeds on itself as a chain reaction. Stainless steel is good when it is kept clean and dried within a short distance of time, however if any oxidants which can potentially attack it, will without mercy.. Remember that stainless steel is steel consisting of > 60% iron which is not an inert metal and will rust once the passive layer is lesioned. If this bothers you so much as it does for me, I would pass on the sale and look for something different unless the price is unbelievably irresistible.
The pits seen in your example seem most surprising as the most common places you are likely to find pittings is;
(1) between the case back and inner case back gasket groove which you would need to open the case back to see properly
(2) under the bezel - both on submariners and other oyster cases
(3) between the lugs.
In my opinion these areas should be coated with a more inert protective layer like a gold plating as they are near impossible to access for average watch owners.
see my previous write up on