A Project Box should be used where the profile is *managed* by a Project, not just belongs in the right time frame or geographic area.
For instance, a profile who lived in England could only have the England Project Box on it *if* it was being managed by the England Project. If it was being managed by anybody else, then - no England Project project box. If it was being managed by, say, the Notables Project, it could have the Notables Project project box. If it was being managed by me, then - no project box, because I am not a Project.
The implications of there being a project box but no project: well, you aren't going to get struck down by forked lightning, but sooner or later someone's going to come along and say "that shouldn't be there".