The undamaged fabric is airtight, so you definitely have at least one leak somewhere. The first question to consider is whether or not it’s worth your time trying to track it down. Based on some quick math and an average lung capacity, the rate of air loss you’re seeing is equivalent to roughly 1/2 litre (1 pint) per hour, or 8 ml (0.25 oz) per minute, which would be difficult to detect, but not impossible (maybe one pea-sized bubble every five seconds). This rate of air loss will be vastly outweighed by pressure changes due to fluctuations in sunlight intensity and air/water temperature as you paddle, so even if you find and fix it, it will make no practical difference. That said, it is annoying just knowing there’s a leak somewhere, so if you do want to find and fix it, I totally understand.
For a very slow leak like this, you’d probably need to use a highly concentrated mix of soap in water (maybe half and half) and paint it on and then stare at each area for several seconds to see if a bubble starts to form.
Assuming you’ve tested the entire surface of the tubes, the seams, the valve, zipper, and all the usual suspects with soap and haven’t found the leak, the helium test might work, but it sounds expensive (unless you already have access to the equipment and a large amount of helium) and you’d have to check to be sure the detector can register the concentration of helium you’d expect from a very small leak.
The water test might work, but unless you have a zipper in your packraft I wouldn’t recommend it because it would take a lot of work (many many inflation/deflation cycles) to dry out the inside of the packraft. Water has pretty high surface tension and higher viscosity than air, so after adding water, pressurize the packraft with air. It will leak even slower than air and you’ll be looking for a very small amount of water.
Personally I wouldn’t go the latex route… I have no idea if it would work but it would definitely discolor the fabric and increase the weight of the packraft significantly.