Water heaters don’t have air pressure, so that isn’t the problem. They function by water pressure and if you had that kind of leak, you’d know it.
Could you more fully describe how your tank is installed? You mentioned a 3-foot flex line. Nothing that long is made for water heaters. Perhaps you meant a 2-foot flex? And apropos to that, do both sides have flex lines connecting the water heater to the plumbing? Is your plumbing galvanized or copper? Are the nipples in the water heater steel or brass? Did you do the installation or a plumber? If the nipple are steel, is it possible plain galvanized could have been used?