So sorry for your loss, I can relate-- I am dependent on my 9 year old to shut the coop door at night when I am at work (DH has trouble fitting into run door opening to close it, majorly bad coop/run setup it was our first coop-- sometimes I think you should have chickens for 6 months before building your Coop!!!)-- and she forgets to close that door occasionally.....(only saving grace is that our dogs are loose at night outside of the larger fenced chicken area, so hoping that will be a second line of defense...)
I have read on this forum that your missing chicken has a good chance of coming home in a few days, sending Good Thoughts your way.... on that....