This is the easiest signal though, on basically any account. You can see the time that communication happens, and the times when it doesn't.
For example a while back I wanted to map out my sleep cycle and I found a tool that charts your browser history over a 24 hour period, and it mapped almost perfectly to my sleep / wake periods.
But keep in mind that other less obvious data sources can often lead to similar issues. For example phone accelerometer data can be used to precisely locate someone driving in a car in a city by comparing it with a street map.
In the context of the military even just inferring a comprehensive map of which people are on which shift and when they change might be considered a threat.