"a few surface rust spots" ?
Something that gets hot (exhaust) and will be exposed to rain or snow or salt and other road grime will somewhat tarnish on the surface.
Had you kept your stock exhaust system on teh car, by now it would have looked similar, given your driving pattern.
It's all just on the surface. Stainless Steel does not rust through for many many many years.
If you're that paranoid about how your pipes look underneith your car where no one can actually see, why not get some "steel wool" and polish it yourself? You'll quickly see two things from this exercise:
- It surely is just surface tarnishing, and some elbow-grease polishing will return it to luster, until the next month when you need to repeat the process again! Wait until the "drive by wire" aluminum throttle body starts to oxidize on your car, are you gonna complain to GM that it's "tarnishing" when the elements start to react with the aluminum there?
- The "important parts" -- i.e., the tips, are more expensive T304 stainless which stays shiny but is more costly to produce. The tips are all most people see. These are very easy to polish and keep clean (no steel wool here; I personally use "NeverDull" and the results are spectacular, on the tips!)
Are you one of those people at car shows that jacks up half of the car lengthwise and puts huge mirrors under it? If so, why are you driving the car in anything other than 100% sunshine and dry weather?
Sorry if this is taken harshly, but do you have an actual problem (i.e., hole through the exhaust system, break in one of the welds, unexplained black carbon leaking out from one of the connections?) .... or are you just trying to learn how exhaust systems "weather" over time?
Mine looks similar, but honestly, I'm not worried. I have no leaks, holes, or broken welds in my setup. If I ever do, I'll have it taken care of. Until then, nothing to worry about, and yes, it's "Just minor surface oxidation" on the stainless T409 exhaust system.