Yes, that is Sachiko’s box seat. (No, it’s not a permanent installation, but we’re in no particular hurry to get rid of it.)
Lately Watanuki has been waiting for her to settle down in the box before he walks up and looms threateningly over her until she leaves. Then he settles in and gives her a mock-innocent look.
All very 1930s gangsterish. It’s not by accident that one of his nicknames is “Thugbutt”.