In my experience dex-cool tends to evaporate or dry up from engine heat before hitting the ground with a slow leak. the best way to find it is with a pressure tester and let it sit for about an hour or until you can see it dripping. I have seen a slow leaking intake manifold cause this problem a lot, but more before the gen 3 motors. I doubt it is your heads. i would say intake gasket or possibly water pump. see if you can get under the vehicle and see dried up coolant on back of motor.