To the best of my knowledge a dog showing real aggression will often have its tail pointing straight back and some dogs the hair on their back will be up. Fear aggression is often accompanied by a tail between the legs during the growling.
Confused aggression (with the dog not knowing if it should attack or retreat) is often accompanied by a tail wagging during the growling and snarling.
The dog at the pound sounds like it is confused and missing its owner. It was probably fear aggression caused by unfamiliar sights & sounds and lots of strange people looking at it. It has probably used the defence before when strangers go near it. The dog growls so the threat (people, other dogs, etc,) go away.
I have seen a number of dogs that didn’t like women, and some that didn’t like men. A Jack Russel that was being babysat by a friend would not let any female touch it, yet any guy could pick it up and do whatever to it. Some dogs don’t like coloured people, some don’t like people with facial hair. We have a dog in our group that I have seen almost daily for 5 years, and she still won’t let me pat her. I can pat every other dog there but not this particular one. Other people in the group can all pat her, just not me.