I don't have a sun roof, and the shelf below the wipers has been taken off and old leaves or crap has been removed, there wasn't much anyway. I'm not car savi but on looking myself I can see the steering column? above the pedals, could something be coming off that?Do you have a sunroof? If so, the drain is probably clogged or has become detached.
You have to find out if it's oily or just dirty water. There's no oil that can leak into the driver footwell that I know of. If your mechanic can't figure it out then idk what to suggest. Maybe you just spilled something on the floor.