The rules and the precedence of the Chair are quite clear. When the Chair asks for unanimous consent on this or any other motion, it is not necessary for the members to be in their places. It is only necessary for the Chair to recognize a member anywhere in the Chamber. That can be behind the curtain. It can be anywhere in the purview or the sight of the Chair. It can be anywhere that the Speaker can see that the member is a sitting member.
In order to vote it is a completely different story. Members must be in their seats at the time the question is read. It is an interesting and a good question, but that is the reason.