Unfortunately it's a weak point by the design of the plastic sill covers. It's not sealed together with the body at the ends (close to the tyres), thanks to it's shape the mud can collect there, the tiny drain holes get stuck really easily, all of these leading to the above. An even worse example:
http://www.sportsmaserati.com/showthread.php/11867-Ghibli-2-restoration/
The plastic sill are quite easy to remove, just remove the screws at the bottom and the both ends, to access the later ones the tyres have to be removed. Then with some force pull down and rotate upwards.
So as a prevention I think it's advisable to remove and clean the area yearly.
Mine is from '95, with 140k kms. Only with me since 2 years, always garaged, never used during the winter, also trying to avoid rain (today I couldn't).