You said before that she likes people, so she probably doesn't have an irrational fear of them. She is just being alert. A fearful dog cowers when people approach. This breed is not supposed to be instant friends with everyone, not like a lab or a golden. They are supposed to be reserved. Mine did this at a young age too. He goes out when there is lightning and thunder, he goes out when there are fireworks, he followed me into the water his first time at the beach, and he approaches dogs bigger than him at the dog park and hangs out with them. He barks when the neighbor next to me walks his dog, but when the neighbor comes up to the fence to pet him, he wags his tail and licks his hand because he knows him and likes him. He is not afraid of either of them. He probably just barks in excitement. I would just call it alertness. You can count your blessings because she will continue to be a good watchdog. Other Bullmastiffs don't display this until later, and some never display it at all. You don't have to train her to be a watchdog. Just continue to socialize her so she can properly judge situations.