Eventually, I saw a tiny slip of dried blood on the newspaper under the watering tray.
The entry point into the brooder was through an unsecured length of chicken netting and looking around I saw several places where the predator dug little holes in the dirt around the brooder. I suspect a mink.
After I took an inventory, I found that I lost three chicks including two black ones and another New Hampshire or Rhode Island Red.
I fed the remaining five chicks, cleaned the water tray, and secured the wire netting around the entry point. I covered another possible entry point with additional netting.
I moved the live trap from the garden to a spot under the tarp behind the brooder. I looked unsuccessfully for a rat trap that is hiding somewhere in my cluttered house.
My emotions led me to wax philosophically as I thought that everything has its natural duration. After a few years of raising chickens, I come to expect loss and short periods of time with these birds.