This is what I think too - why offer up that info - going up stairs.... He didn't say - open the door to leave - cause that is an action - just like going up the stairs is an action.Something happened in that house that he is with holding.
I keep going back to his initial statement. He said she went upstairs, came down and then went out to clear her head.
He tells us she went upstairs, he doesn't say why, and then came back down. In the greater scheme of things, this is extraneous information. Yet, he finds it important to put it in his statement, why?
What did she go upstairs (if she did) for?. This is where there is missing information.
Did something happen on the stairs? While upstairs or after she came down.
Again, telling us she went up and downstairs is needless, but important to him.
He specifically mentions stairs - I feel this is because stairs plays a part.