If a combadge were to be continusly transmitting locations and medical info, that would make it easy for anyone to tract it (ie "bad guys"). Even if they couldn't read the signals because of encoding, they would still know the location of any starfleet combadges on the planet - or aboard their ship.
You're not always going to know if someone is looking for you. Better to activte the combadge only when needed.
When you ask the computer for someone's location, I think the computer calls the 'badge and the 'badge sends out a location beacon.