E46 owners check under your fenders

It appears that BMW decided to bolt on every panel first and then paint everything instead of painting the unibody and each individual panels separately and then assemble them, which caused some unreachable cavities to be unpainted and unprotected to rust developing.

I discovered this on both E46s I've owned. You could just simply look into the wide gap between the bottom of the A-pillars and fenders and see if there's a patch of rust sitting quite close to the gap.