I had a look under the rubber seal of the boot a month or so a go and saw some rust patches, especially where it was welded. See below. There was nothing to note along the sides. The last photo is a previous one that I took when undersealing behind the bumper but you can just see the rust patches where I have removed the rubber boot seal. A fair amount of the rust is actually rust stain that has crept along under the seal as is seen in the middle photo.