I noticed this when I was building a new rocket and forgot to attach some automation to a Gas Output Fitting.
It was running despite no gas being in the rocket's storage.
Liquid Output Fittings do this as well (went back and tested it) when no liquid is present.
Build a Gas/Liquid output fitting on inside a rocket (one with some sort of power source) and don't have gas/liquid in the rocket's storage. Observe that the Gas/Liquid Output Fitting is still using power and generating heat despite not doing anything.