It will be rats WarescotFarm. Although another chicken is possible it is the least likely by miles- mum would have fought her off. We have 12mm weldmesh around and over ours and they are kept isolated with mum when applicable. Any bigger mesh and the chicks can get their heads through and then get bitten off. Gamekeeper friend had lots of chicks with no heads and couldn't work out why. So he watched and waited. Seems the rats frightened the chicks at one end of the pen and they stuck their heads out of the wire at the other end trying to get away and another rat was quietly waiting.
We don't have a base mesh because I am worried about little feet getting caught in it. So we move the run and coop daily and keep it well out in the open. The top also has plastic sheeting over the mesh for shelter.
So next stage before hatching again is a purpose built broody run and coop -a large rabbit hutch and run may do the job if the mesh is small enough. They also may need wind protection around the outside bottom.