Our Hobbs meter runs just as described above. Power from MRGB pressure switch and activated via collective position switch. Depending on thresholds set, with collective full down, you could be flying and not spinning the Hobbs. Same could take place with tracking system. Unless you fixate on the gauge, you really wouldn't know until post flight and comparison of hours. Would seem more logical to have it on a Battery Bus. We use the spidertracks and it's hooked into our avionics distribution. So power up the avionics and it comes on. You can see at anytime during the flight that it has power. You really don't know if it's tracking but experience has shown it works 98% (my estimate) of the time.