If my dogs were growling at "something" i wouldn't really worry about it, they hear and smell things so much better than we can, we wouldn't have a clue. If they directed their growl at me, or someone they know, yes, they would be in the vet until something can be found. There's so many things that can cause aggression out of the blue, even something like Lyme disease. Epilepsy too, but pretty sure there's nothing that can test for that.