The Mysterious Case of the Disappearing Lemonade

Who dun stole mah lemonade?

Yes: I am posting a picture of some lemonade. But do not adjust your set just yet, for it is a bottle of lemonade that I find quite interesting*.

The lemonade level, as you can see if you squint, is just below half way up the bottle. This is not odd in itself, but requires the following extra snippets of information:

• The bottle was bought in a markedly more full sort of state.
• The bottle cap is still sealed (i.e. it is still attached to the little plastic ring that lets you know if some scrote has opened it in the shop).
• There are no readily identifiable holes, cuts or other sorts of damage to the bottle.
• There are no stains, splodges, or stickinesses in the vicinity of the bottle's storage location that may indicate leakage. 
• The use-by date, in case you're interested, is August 2010.

My primary hypothesis for this occurrence is that the gas dissolved in lemonade and other fizzy drinks in order to make it fizzy has become undissolved. It makes sense that this happens over time- that's how fizzy drinks go flat. What amazes me is the difference in volume. For comparison, there were other bottles in the same cupboard from a similar period of recent history which hadn't changed noticeably in terms of lemonade-level but, when opened, were experienced to be as flat as the proverbial. These were, however, of a different brand.

So I put it to you, my freaky friends who enjoy a conundrum or two: Where's my lemonade gone? Do you think my idea holds water**? Or is something else afoot***?

* You, of course, are free to disagree with me on this. And probably will if past indications regarding things I find interesting and things other people find interesting.
** We've already seen that it doesn't seem to hold lemonade.
*** You may deduct 20 credibility points for each occurrence of the following in your proposal: aliens; 2012; apocalypse; Dr Gillian McKeith; dowsing; Mayans; spiritual; magnetic pole reversal; galactic conjunction.