Only two things I can think of:
1. He died, and the other fish picked at him enough where there wasn't much left by the time you went looking for him.
2. He jumped out of the tank. Not sure if hatchets are known as jumpers, but I had a couple fish go missing on me, turns out months later I was dusting around the fish tank and found two shriveled up fish sitting on the ground in the 1/2" of space between the tank stand and the wall. (Only opening they could have jumped out of was about a 1" of space where the water from my HOB
filter comes back into the tank. For a fish that's a jumper, that's enough!)
P.S. -- I agree with what's already been said, if there is a discrepancy between what your heater says and what your thermometer says, trust the thermometer. I have a really nice, expensive all-stainless steel heater in my main tank, in order to keep the tank temp at about 78 I have to have the heater set for 73.5. Go figure.