if the you cant even tell if there is a pouch i would say its probably not gas in the pouch (it would be pretty obvious), and since you haven't said anything about external signs it probably isn't external gas bubble disease either...so i would lean towards internal gas bubble disease.
the internal form is caused by one of two things, gas supersaturation or bacterial infection. if its bacterial infection it can be treated with antibiotics as i mentioned above. if its due to supersaturation the treatment process is tricky and involves keeping the seahorse at a depth around 4 times the tank height for a couple of days. its similar to the bends that human scuba divers get when nitrogen comes out of solution in the blood. so this is like putting the seahorse in a decompression chamber (high pressure) and bringing them back to normal pressure over the course of a few days.
im no seahorse disease expert (i did stay at a holiday inn last night), however i would treat for the bacterial version first. like i said ive never had to do either of these treatments, only information from what ive read.
hopefully some of the seahorse members who have first hand experience with these diseases can chime in on this.