The complaint is seasonal. Through summer the tip and the rear section stay bright, then the first salt goes down and a brown speckle appears across the outside of the pipe and on the face of the tip. It washes off, the car runs clean for months, and the next winter it comes back in the same places.
Seasonal recurrence is information, not bad luck. Material that is wrong is wrong all year. Something that arrives with the cold and leaves with it is being driven by an outside condition, and there are only two candidates: chlorides from de-icing salt, and the fact that in winter nothing on the underside of a car ever dries.
De-icing salt is a chloride, and chloride is the one thing a passive stainless surface cannot tolerate in concentration. It breaks the passive layer down locally, and once a pit opens, the chemistry inside the pit keeps itself aggressive and keeps working outward from a point rather than spreading as a film.
The second half of the mechanism is drying. In summer, road spray evaporates off a hot exhaust within minutes of a drive ending. In winter the car sits at ambient, the panels stay wet, and a salt solution stays against the metal for hours after every journey. The car is being held in the one condition the alloy dislikes.
The most common winter finding is not corrosion of the stainless at all. Grit thrown up by traffic contains particles of ordinary steel from studs, chains, brake discs and road furniture, and those particles embed in the surface. They rust. What you are looking at is a speck of rusting foreign metal sitting on sound alloy.
Separate the two by shape. Free iron shows as discrete brown specks and small comet trails, evenly scattered, usually heavier behind the wheels and along the lower faces. A genuine chloride attack begins as a rougher bloom with a pit at its centre, and it prefers a low point, a crevice or a weld rather than an exposed panel.
Two other faults wear the same face. Sensitised weld metal rusts in a narrow band set back from the seam, and it starts at the joint and stays there whatever the season. Galvanic attack where stainless meets a plain steel bracket or a coated fastener rusts at the contact, with a clean tide mark around the affected area.
Both of those are permanent and both are visible in high summer. If the staining disappears with the salt, or if it only ever appears on the outer skin and never at a joint or a fixing, the material is probably sound and the exposure is the problem.
Clean a patch with warm water and a soft cloth. Free iron wipes off as a brown smear on the cloth and leaves unmarked metal beneath it. A pit does not: it stays as a dark speck with a raised rim, and a fingernail catches on it when you drag across.
Then take a small magnet to the specks. It will not pull on the stainless, but it will pull on an embedded steel particle, which tells you the rust was imported rather than generated. Finally, look at where the staining stops. A line that follows a bracket, a shield edge or the inner lip of a tip is telling you exactly where water was being held.
The practical answer is to remove the salt before it sits, not after. Rinse the underside after a salted drive, including inside the tip and around the hangers, and then drive the car long enough afterwards for the system to dry itself. A car that only does short winter trips never gets that chance, and the exhaust pays for it.
Finish quality still decides how much margin you have. A brushed or polished surface with no embedded iron and no weld oxide left on it holds its passive layer far better than one that was only wiped after fabrication. It is worth asking how a stainless steel exhaust tip was finished, and whether the rest of the stainless steel exhaust system was prepared to the same standard underneath.
