Based on my experience with a 2002 X-type and the rotten sills under the plastic covers I decided to take a look under the sill covers on my 2008 S-type 4.2. This is a Canadian car that sees winters so I guess the results was to be expected. I was most surprised though at the rusted fuel line brackets which are all under the plastic undergrads, I would not thought they would almost be rusted right through.
I have painted the undersill area with POR15 and got some rust check spray to the inside of the sill so that it runs out through the layered, spot welded seam. Hopefully this will save the area from the disaster that befell the S-type. If you have a northern car and have never had the sill covers off I would suggest it as good preventative medicine. Ruster lower sill lip, 2008 S-Type

 

Thanks for reminding me, that's another priority job before next winter.
I fetched out an alarming amount of mud, silt, clag and rusted metal flakes from behind the inner wheel arches behind both front wheels.