I live in a household that includes 4 people and 5 dogs. Of this group, one of the people and 3 of the dogs are relatively new: a new roommate moved in and brought a breeding pair of min-pins, and we kept one of the puppies from their litter.
The youngest dog, a min-pin puppy, developed the habit of barking at one of the people who lives in the house. Just to clarify: the puppy is just over a year old, was born in the house, and the person that sets her off has lived here for almost 5 years. So he has been a member of the household since she was born. For some reason, this one dog has focused on this one person, and every time she sees him moving around the house, she goes into a fit.
Typically, he will leave his room and come into one of the public areas (kitchen, living room, den, etc), and she will bark very angrily at him until someone calms her down. It typically requires someone to approach her and calm her down before she stops -- she has never responded to voice commands when in this state. (I will note that she's never aggressive when in these moods -- no growling or biting, etc.) After that, as long as he remains in a public area, she exhibits normal behavior towards him. But as soon as he leaves (goes to the bathroom, or back to his room, etc.) and returns, it starts again.
I know the solution to unwanted barking is to try to train her to do something else (incompatible) instead, but I'm at a loss trying to figure out what is triggering her behavior in the first place. As far as I am aware, he has never done anything to her that would make her act this way. She sees him for many hours a day every day of the week; he plays with her, pets her, gives her treats, takes her outside to do her business, etc. Sometimes he's even the one approach her when she's barking and she will calm down, though other times she will backpedal if he tries.
What might she be picking up on -- that she doesn't pick up from the other 3 of us, and that the other 4 dogs are ignoring -- and how can I train her to react differently to it?