I was doing a few jobs today one of which was fitting new mudflaps, whilst fitting them i undid the clips at the rear of the front arches and noticed some mud behind the plastic trim so removed it and the plastic sill guard, i scooped out over a kilo of wet mud from behind the rear of the front wing and a fair bit of mud from the front end of the plastic sill trim.
I would guess this could be a major rot problem if left. Hopefully i wont have the problem again as i now have big mudflaps covering it all.
I covered the sills with waxoyl and replaced the trim to try and protect it for the future.
Might be worth taking a while to check all yours.