There isn't a separate Mentor licensing process on Windows. If the application and it's child processes exit properly, the license should be checked in. However, a spotty network connection could interferre with the client/server communication during the exit process. Is this client connecting to the license server through VPN? It sounds like the server is returning the license to the pool after a timeout period of not hearing from the client.
The fact that the license is returned via a reboot of the client tells me two things: all application processes are exited and the socket to the license server is cleared.
It really sounds like there is a networking issue when freeing the license and/or trying to reconnect to the server. This is probably something you should open a Service Request for if this advice doesn't help find the cause.
Thanks, I was thinking it was more of a network issue. It has not occured again this week so I will need to continue to monitor.
Qualcomm Technologies, Inc