Hmm, one thing that occurs to me is that the added juice may have been in very small print on the label. The liquid would still look perfectly clear for the same reason that the 2% of lemon juice in most standard UK lemonade, doesn't alter the clearness if the finished product. Yet even 1% juice would add calories.
Food regulations don't permit a very long shelf life for soda water and other mixers these days so old stock doesn't really exist. Besides the plastic gauge they use, doesn't allow the retention of the carbon dioxide for very long. Yes, glass bottled soda water does exist, but it's usually Schweppes or the like and is a pub sized 200ml bottle. They've never marketed a low calorie version for the reasons I said earlier.
If it does have juice added, I suspect the FSA would be very interested in the label. The key here is the declared calorie content per 100ml, a legal requirement.
I wonder if the label declares "< 1 calorie" on the label nutritional data table yet has zero calories elsewhere on the label. That would be a different issue but I'm getting into food regulation technicalities here .