You didn't say whether this was on the interior or exterior of the body or the door itself. So if the current owner will let you, why not remove the two bolts and see what's underneath?
If it is rust, (could be) then if it's on the door body itself, you might be able to get a new door; have the bad area cut out and replaced; use POR-15 from what a neighbor tells me (haven't tried it myself) and then fill it with their cloth, or put the plate back on.
If it's on the body, if on the exterior, ooh bad, don't like that, sounds like someone did an el cheapo fix on the rocker panel rust or dent. If it's on the interior, then again could be rust, or could also be used to hide the access holes for the rocker panel support part of the passenger area floor pan (if on the side of the floor pan right beneath the door) If on the floor pan itself, then could also be rust, and a quickie repair instead of a cut out and weld.
Bottom line, either ask the owner, or see if you can remove and inspect.