How is a 25-year-old can of soda now empty without having been opened or poked?

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.

I’ve had numerous experiences with unopened old cans of soda losing part of their contents, though my cans were no where near as old as yours. I do remember one being empty.

The snap top tabs probably don’t make a perfect seal. That combined with the positive pressure of the carbonation might result in very slow escape of the carbon dioxide gas, probably also increasing the size of the leak. Thereafter there’s probably slow evaporation of the contents over many years.

Why the Coca Cola cans didn’t also lose all the liquid is another matter. But coke has a reputation for being very acidic. People are known to use it as a cleaner. I also read it interacts with aluminum causing corrosion. Just a guess but perhaps corrosion somehow improves the seal of the snap tab. A corroded bolt is harder to remove.

Hope this helps.

This also happened to me with an unopened 7up can from 1993 which I had stored in my collection. When the can was about 6 years old I found it looking intact but feeling completely empty; underneath and around it was a small puddle of sticky clear goo, hinting that its contents had very slowly leaked through a tiny, indiscernible hole somewhere along the can's bottom. The water probably evaporated during the process leaving just the syrup. It's very likely that the same happened to you, except that in your case the can was untouched for 25 years - surely enough time for the members of some lucky ant colony to loot the fine tasting sugary goo and disappear without a trace.