My dog will only pee in the house when she is left in the open space. Why would she do this?

My dog, a 6-year-old tripod staffy mix, will pee in the house while everyone is out, no matter how long it is. 5 minutes, 10 minutes, doesn't matter. However, it is only when she and the two other dogs at home have the run of the place. If she is put in the laundry room (a fairly big space with beds, food, and water), she will be perfectly fine. We had a crate for her early on, but she got incredibly stressed while in it, to the point where she would try to chew and bend the bars of the crate. So the laundry room has been working well for her when we need to go to work. We have a bathroom schedule for the dogs, and while we sometimes have to physically carry her outside because she won't budge, we won't let them back in until they go.

Any ideas as to why this could be happening? I thought it was stress from the openness of the space, but she only ever pees if there are no people around. I'm so confused, and I want to be able to help her since it is stressful for everybody.