Hmmm, maybe someone has been "admiring" your car when you weren't around?
Sorry, couldn't resist.
There should be nothing in the sidesills that would cause this. Are you the original owner? My guess would be that someone probably put something inside the sills (either heat paint or header wrap) that isn't working, and it's liquifying and dripping out.
Pull the sill and take a look - it's easy to do. If the paint is bubbling, you can *probably* get it fixed under warranty. Others have, but others haven't also.