Are you able to post a picture of the area in question that’s showing signs of corrosion?
I can’t remember reading of other instances of corrosion but that’s not to say there haven’t been any - I seem to remember reading your car’s a 19 plate so it’s around six years old, so I dare say there will be some cars of a similar age to yours with corrosion, e.g. those that spend much of the time being driven on poorly maintained roads and any resultant stone chip damage has been left un-repaired etc. (checking for - and repairing - stone chips is part of my weekly / fortnightly car washing routine to stop the onset of corrosion)..
The area you’ve described that‘s showing signs of rust is directly behind the front offside wheel so will be subjected to a certain amount of ‘abuse‘ from stones / other road debris, spray and road salt thrown up by the wheel. So if there’s ever been any stone chip damage in that area that’s not been repaired, then it’s quite possible corrosion could start.
Edit; alternatively, maybe the area of the wing showing signs of corrosion has had previous paintwork repairs that weren’t carried out to the same standard as the factory-applied paintwork. Your detailer ought to have a paint thickness gauge; assuming they do, they could measure the paint thickness on the lower wing and compare it to the paint thickness on surrounding panels. If the paint on the lower front wing is noticeably thicker than on surrounding panels, that would suggest it‘s had previous paintwork repairs.
I doubt the corrosion on your car would be covered under Seat’s corrosion warranty as the warranty would only cover corrosion resulting from manufacturing defects (even then, there‘ll be warranty exclusions); there won’t be any cover for corrosion caused by an ‘external influence’ - e.g. where corrosion is the result of stone chip damage to the paintwork caused by loose road chippings flicked up by the wheels.
Haven’t you only very recently bought your car? If you bought it from a garage, you could always see if the dealer would pay for - or contribute to - the cost of repairing the damage. If they won’t, then IMHO it’s important to get it fixed sooner rather than later to stop the corrosion becoming a bigger
problem.
