Why does Coca-Cola Zero Sugar Contain Sodium Despite no Sodium Ingredients? According to the Coca-Cola website and every label I read, Coca-Cola Zero Sugar is a mixture of organic compounds: 


  CARBONATED WATER (OC(OH)₂), CARAMEL COLOR (Various sugars), PHOSPHORIC ACID (H₃PO₄) , ASPARTAME (C₁₄H₁₈N₂O₅), POTASSIUM BENZOATE (C₇H₅KO₂), NATURAL FLAVORS, POTASSIUM CITRATE (C₆H₅K₃O₇), ACESULFAME POTASSIUM (C₄H₄KNO₄S), CAFFEINE (C₈H₁₀N₄O₂).


The label also says 40 mg sodium. There's a perceptible salty taste in the beverage, which suggests to me that something activates the taste receptors for saltiness, but a potassium cation can do that just fine.  

There is no sodium in any of those molecules. That means "Natural Flavors" must contain the sodium... but in what form? The essential oils in things like OpenCola seem to all be hydrocarbons without sodium. And if it's plain old NaCl table salt, why wouldn't they just list it as "salt"?

Where's the sodium in Coke coming from? 

EDIT: The discussion has centered around the sodium content of "still" drinking water; what about the process of carbonating the water? Some processes add a sodium carbonate to the solution; could that be the source, contained in the ingredients list as "carbonated water"?
Two sources: water and natural flavors

According to the Wisconsin department of healt natural sodium levels in water is $\pu{40-60 mg/L}$ meaning a $\pu{20 fl oz (\! 591 mL)}$ Coke has about $\pu{25 mg}$ of sodium from water. Thus the other $\pu{50 mg}$ of sodium must come from natural flavors. This makes sense since Coca-Cola uses plant products like coca leaves which would naturally add sodium to the carbonated sugar water.