Well, it's his bank, so the lobby really is for his use to some degree. Also, I'd have to think they'd pay attention to who hangs out in their lobby repeatedly

Well, it's his bank, so the lobby really is for his use to some degree. Also, I'd have to think they'd pay attention to who hangs out in their lobby repeatedly and for any length of time, if only for security reasons. (Or I'd have to hope.) As long as he's a customer and doesn't seem to pose a problem, they probably don't mind. (And bankers would do something if they perceived a problem, if only by having the security guard hang out very close to the people in question.) If they haven't noticed that a few people work in the lobby, then those people really must not be impinging on anyone there.