I would buy a trail camera and carefully watch my remaining chickens just in case.
A fox can easily climb a fence. Normally if there's no evidence, it's a canine of some sort, fox or coyote. I lost a hen between noon and 4pm to a fox.
Hopefully she returns though.

My favorite chicken of all time was my silkie rooster, who the fox took...that's when I had to stop letting them outside the barn area. Of course the hen was inside when he climbed over that fence, which had never happened in 13 yrs of living there.