Possibilities ....
Water pump shaft seal, oil cooler if you have one, pin hole in exhaust manifold (between gas side and cooling water side), weepy head gasket, etc. etc. Sodium is a trace contaminent in virtually everything on this planet. Could even be a natrual contaminent from the oil you use or from natural blowby of the compression rings, etc. I wouldnt worry about trace amounts of sodium. Putting this in persepective if you layed a sheet of plywood on the surface of the state of Connecticut and compared its area to the whole surface of Ct. .... the 4x8 sheet of plywood would be a 'trace amount'. ;-)