An excellent question to which I don't know the answer. Regrettably, the answers are probably behind the aforementioned paywall :(.Where did the CO2 go (the bladder isn't designed to hold gas, so it can't have gone unnoticed)?
ETA: Layman speculation powers: ACTIVATE!
I can only think of two vectors: Either the bloodstream or the urine itself. CO2 is a natural byproduct of cellular metabolism, isn't it? It goes into the bloodstream and thence via veins to the lungs and out to melt the planet. That's one route. The only other route I can think of is into -- and out with -- the urine. Urine changes color from deep yellow to near transparent based on the trace elements in it -- could CO2 be one of them.
Layman speculation: DE-ACTIVATE!