Why does Coca-Cola Zero Sugar Contain Sodium Despite no Sodium Ingredients?
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.