My wife had the same color on hers and in 30k miles, 1.5 years I never saw anything like this happen. That's really odd..let us know if you find anything on this.
Sorry to hear/see that, gotta be frustrating! Did you remove or bypass your cat? If not, have you tried using a different fuel brand/source? Can you tell if your car is running too rich?
Running rich should leave a black residue if anything. The yellow is what throws me off. Old clear coat will sometimes look yellow on a white car but the fact that it can be removed and it comes back. I see why exhaust comes to mind but I can't see it turning yellow from that. weird...
It would be interesting to see a Gen X Accord in a wind tunnel as I've noticed that area of my bumper collects dust/dirt/debris after driving. My thought is this is a dead space area and the reason for this occurring and is more visible on a white car.
The fact it comes back so quickly and profoundly is what really has me stumped. I initially thought maybe its pollen, but ive been able to rule that one out.