When viewing the details of the user count by license type for "Full" license users, it appears that users show as having logged into PPM when it is known for sure that those users did not log into PPM on that environment.
CA PPM 15.x and up
Through investigation, it was found that those exact users who showed as having logged into the system had initiated workflows in the system, and had action items that were waiting. Once those in-progress workflows were cancelled, the user count by license detail no longer showed those users as having logged into PPM.
It is confirmed that this is working as designed as the process engine heartbeat in PPM is set to check for changes on those in-progress processes/workflows at a heartbeat of every 60 minutes. Thus, those users who initated those processes/workflows were showing as having logged into PPM every 60 minutes. This does not affect performance at all and will not cause issues as system usage grows. The application is doing this by design and will not have any effects on the usage of the system.