Ahh you're right. I tested the same amount of time with him just standing around base doing nothing and he didn't get as tired. I suppose activity affecting tiredness is new and I just didn't notice it in the notes. It's not a bug, it's a feature!
I started up a sandbox file with zombies set to none, so I could mess around and build without having to worry about fighting. I wandered into a house, and to my surprise, there was a zed in the kitchen. I got into a car and began driving around while honking to draw any stragglers out- and sure enough, I've drawn out a little conga line of about 7 or so.