QC for process inspections is usually different people than final product inspection.
My guess is that those spots simply do not show up until a few weeks to months after shipping, and QC has no procedure for accelerating and inspecting future rust through.
What this points to I think, is a failure in the Manufacturing process. Somehow a batch of frames missed a passivising step or were not in the bath long enough or the solution was contaminated or not checked and changed on schedule.
So many things can go wrong that have nothing to do with QC. Unless you want them inspecting every work station every hour you are going to get things like this happening. With that kind of QC you can probably double the cost of your new SIG.
Fact is, people eff up. Even people employed by SIG.