It is definitely not some sort of drain... just the bottom edge sheet metal foled cver in the corner. That area is not protected from the elements, so water will collect in that small space, therefore cause rusting, I advise all owners to clean then seal that little open corner with silicone, I went one step further and added a seal to dress up the area, here are some pics...
seals added to underside of doors, completely seals sill area when door is closed, can't see seal unless you get down there and look. They are inset about 3/8 of an inch...stays so much cleaner down there!!
Sorry crappy photos, think I'm buying a Nikon SLR tonight!!!