2 1/2 inch wire is huge for chicks. They could easily have popped out through it and been taken, but equally a stoat, or more likely a rat will have got in and taken them.
We had Muscovey ducklings in our greenhouse one winter, with half inch mesh on the floor, but some still disappeared. We found it was rats underneath, pulling them down through the tiny mesh. You wouldn't think it possible, but we saw the evidence, and some bitten ducklings which survived (for a bit). Eventually the terriers dug out a HUGE rat which had set up home underneath the greenhouse, probably just to feast on the ducklings.
So you need much smaller holes in your mesh, or a solid lining around the bottom.