You can't assume that people who are not on their laptop all the time aren't working. I frequently research stuff on my own devices as the web proxy at work blocks too much stuff.

I also have slow days where my energy is low, and intense days where I work longer than normal. People are not machines. Regardless, I get all the things that need to be done done, and usually a bit more on top.

You should measure productivity, not laptop utilisation.