First look at the Configuration Data tab -> Node View, then sort on the first column usually filled with the green check circle. This will sort the list based on the icon, and any Commissioned units not reporting in will be shown with the binocular icon. You should click twice if you want the missing units at the top of your list. The number of binocular icons will match the count next to the binocular icon in the Site Health section above the Network Tree pane.
A unit is considered missing if there has been no communication from the device over the period of time specified when you downloaded data. If you only just connected to the DCAP, then only 12 hours of data was downloaded. If you manually download only three hours of data to the CIT for example, a unit is considered missing if, for example, that unit was on the edge of its range and succeeds only to get a few messages a day through, or if a Repeater was recently unplugged in the area of the unit. If you download 24 hours of data, often you will find these units are not missing, but just come in less often than once an hour.
In the Monitor Data tabs, if you enable the Last Contact column, you can sort on that column to see how recently a unit has reported in. The time is relevant from the time of the last download that you made.