Corrosion / tin-worm.
There's a serious flaw in this thread. It's light for info on rot and it took the 47th post (Thank you Amazondean) and sdg1970 of this parish to push this thru'. Partly brought about because these cars don't rot, only they're getting to the age where even this adage gets proven wrong. More so if you live near the coast in salt-laden air.
Penny to a pound, you've 20+ years of crud hidden behind plastic. Get the pressure washer out, there's a few mud-traps. Those sills might be body-colour and look like metal, but they're actually plastic. As a rule, much as post 47, where there's plastic trim, sills, wheel-arches etc, the mud collects. This keeps things wet, and here's your rot:
Links:
https://www.volvoforums.org.uk/showthread.php?t=317208
https://www.volvoforums.org.uk/showthread.php?t=302938