How could they have been so stupid as to forget the Cloak? There was no reason on earth that Professor McGonagall would accept for their being out of bed and creeping around the school in the dead of night, let alone being up the tallest astronomy tower, which was out-of-bounds except for classes. Add Nobert and the Invisibility Cloak and they might as well be packing their bags already... what waiting now is a detention.