My guess would be that since you live in Canada, and the car has never been driven in the winter, you have had some mice take up residence in your frame rails. Their urine is acidic and will eat the metal in your frame rails in a couple years. The close proximity of the rails to the converters would invite the little critters to the nice warm place, and then would bake the urine and nest into a nice frame rusting concoction. Is there a dusty, flaky dirt and grass/material, or cotton like fuzz clump inside your frame rails?

It will be interesting to see if the frame rail rust is a consistent with the LX cars.

The fix is going to be like any other frame rail replacement, pull the carpet and everything else off the floor, remove the rusted rail, fabricate a new rail, and weld it back in place. Then replace the removed interior. Gene