The soda can't escape through a sealed can, which leaves two options: the can wasn't sealed or the soda didn't escape.
Soda didn't escape:
This is the less likely and less interesting case: the can was produced defective and never had any soda in it. Issue on the manufacturing line, and whoever saved it (you/parents?) did so because that was novel, and subsequently forgot.
Can wasn't sealed:
Again, a manufacturing defect is certainly possible, and in the intervening years most of the contents escaped through a defect you can't see, probably around the lid or the pop tab itself.
This requires a serious molar flux through the hole. I needed to prove to myself that it was possible. Using Fick's First Law of diffusion,
$$ J = -D\times \frac{\delta C}{\delta x}$$
- A temperature of $20\ \mathrm{^\circ C}$
- Which leads to a density of air of $1.204\ \mathrm{kg/m^3}$, or $0.1204 \ \mathrm{g/cm^3}$
- And a partial pressure of water of $0.0231$
- And a diffusion coefficient of water vapor in air of $0.242\ \mathrm{cm^2/s}$
- About $6\ \mathrm{cm}$ as the distance, given that the can is on average half full and the storage space was very dry
We get a flux of $0.004856\ \mathrm{g/(cm^2\ s)}$
Given that $12\ \text{fl.oz.}$ is roughly $355\ \mathrm{cm^3}$ (and the soda is mostly water which is roughly $1\ \mathrm{g/cm^3}$ at that temperature), that means that a water vapor leak through the top over a period of $25$ years ($7.88\times10^8$ seconds) would only need to be a $0.1\ \mathrm{mm}$ square hole.
That's $1{-}2$ times the thickness of a human hair, on average. I couldn't see that hole, especially in the region under the pop tab. We could say the can was nearly fully empty the whole time and that the room wasn't that dry, with $50%$ relative humidity, and the hole would still only have to be $0.2\ \mathrm{mm}$ on a side.
Can wasn't sealed: (part 2)
The other answer, which I admit I was going to present when the above answer required too big of a hole, is that the fluid escaped as a liquid rather than a gas. Water has strong adhesion and cohesion, and will tend to "wet" any surface it is in contact with. If there was the tiniest hole or crack for water to flow through, it would have done so and evaporated long ago.
As an aside:
Interestingly, colas generally contain phosphoric acid, while sprite has citric acid, which is much weaker (and both have carbonic acid, weakest yet). That means that coke is more likely to corrode a metal container, enlargening a hole. However, aluminum is wonderfully inert in this capacity, and rapidly forms a persistent, impervious layer of aluminum oxide only a couple of atoms thick which prevents any further corrosion, so the sodas should not be chemically etching the can in any appreciable way.
Testing:
As mentioned in a comment above, you can do the bubble test yourself fairly easily. You can even enhance the test by precooling the can (and air inside) and placing it in warm water, creating a pressure difference. You might be able to see bubbles coming up.
For a really definitive leak test, you can have the container tested using a radioactive tracer gas. Businesses exist to do just this, usually connected with the semiconductor industry. They can calculate the exact size of the hole, but such a service isn't free.
