When water escapes a container when the intention was to keep it in the container, we earth folk call that leakage.
Ya got a leak, son…and a darn small one. It might have developed over time, because an imperfection in manufacturing could have led to corrosion over time. See, that there stuff in the can can rust and corrode metals. And where parts meet (can/top, top/lid), well that has to be sealed well enough for a year or two, but I doubt any alum can makers are staying up at night trying to figure out how to keep mildly acidic soda in the can for three or more years.
I have a small, sealed glass bottle of Tequila that my Mom bought me 10-12 years ago from Mexico that has slowly lost liquid over the years. It has never been opened. I lift and move it to dust it, but otherwise it hasn’t been moved. I remember an explanation for where the tequila was going from a physics classes I took a few years ago, but I can’t remember the term…(physics is not my strong point, I remember none of it). It wasn’t just simple evaporation…something about how things can evaporate through glass or probably aluminum, and it made sense when I learned about it…damn. I wish I could remember the name for this.
Maybe the water has evaporated because of small holes in the can. You could open it and see whether the remaining Coke is normal or concentrated to verify.
There is a super thin plastic liner sprayed into cans. Take a knife to the inside surface of most soda cans. That’s there to prevent corrosion from the acidic soda. Its not perfect, it may be thicker and patchy in some parts. At the seam of the hole at the top there’s bound to be some variation in it. A super small pinhole will show up eventually. Probably even so small that you could squeeze the can and the air wouldn’t rush out at any appreciable rate.