Thank you for the responses everyone! Them getting spooked seems the most likely possibility out of the mentioned since they were completely content before this happened and had lots of airflow from the hardware cloth and they looked like they were trying to hide. I'm not sure what animal would...
I don't have photos but there's no trace that anything's touched it. I think there's a possibility it's a predator because they looked like they were trying to hide or get away from something since some were on top on the heat plate and in their hide-out house. Their brooder is see through so...
I woke up to all my chicks [except one week old chick who I found alive squished in a crevasse] dead in their outdoor elevated brooder a bin with it's lid tightly on which is solid plastic with 1/4" hardware cloth. Everything was in place and it doesn't look like anything was there or messed...