COS Burning Question

[bohcoo]

No: HPFGUIDX 78028


How did Professor McGonagall know it was Ginny who had been taken 
into the Chamber? 

No one saw it happen or they would have known where the entrance to 
the Chamber was. 

Obviously no head count was done, or Ron and Harry would have seemed 
to be missing too as they were hiding in the teacher's cloak room.

This is driving me crazy. . . If this has been answered before, 
please clue me in.

Thanks!

Bohcoo