I'm with chipmunk on this. Although you see a 2-wire in the panel, it could be carrying 2 or more loads. Especially bad if they are the same phase. Good way to find out is to put an amp-clamp on it, check the load amps, shut the circuit off, and check again. Another is to shut the whole house off, and ring it out through all of the other circuits with the neutral disconnected. In both ways you could still miss it if the load were not on. The sure fire way is to follow it.
Other possibilities could be, and should not be ruled out, is that the breaker is bad, and it fed several space heaters for a while without tripping. The circuit could have a history of abuse, reset and trip cycles, or one big short in the past. Bad connection further in the circuit, or just plain dirty wire. (Dissimular metal, or chemical reaction, or low grade copper, or buss material.)