How to keep track of how old food is

As far as I know, raw tuna is not usually salted and/or pre-treated.

What may be happening is a case of perceived salinity. For example, there are numerous studies that suggest foods with certain odors can be perceived as much saltier than they actually are. If you have access to that second article (it is copyrighted and behind a paywall, so I can't post it here), it has a great figure on page 3 depicting the perceived sourness, bitterness, sweetness, and saltiness of a number of common foods, including tuna. The aroma of plain tuna alone ranked higher than soy sauce on the saltiness scale, for example. Tuna is relatively high in fat, and other studies suggest that high fat foods can also increase perceived saltiness. Perhaps you were using a fattier part of the fish?

Finally, there is evidence that dehydration increases the saltiness of tuna (which actually seems somewhat intuitive), especially if extra salt is added before cooking. Was your tuna previously frozen? Freezing—especially if done improperly and/or if there are multiple freeze/thaw cycles—can act like dehydration.

There's a misunderstanding possibly of what Heston suggested for the British Navy. He suggested to sous vide it and then store it in the freezer. They couldn't just toss it in the cupboard. Sous vide food is not typically shelf stable. It has been pasteurized - not sterilized.

Pasteurization reduces pathogens to a safe level, not to a 0 level. It doesn't typically reduce botulism spores to 0. Once the food is exposed to normal temperatures, any anaerobic pathogens will start to reproduce again.

To sterilize you'll need to pressure cook meat through to 121 C for 2.4 minutes - at which point its rather not tasty. You might be able to sous vide at a lower temp for much longer, but I suspect it'll still be past the point of being tasty. Sterilized meat tastes like your typical potted meat.