User force logout by system because of session timeout

I’m using yii1. When session timeout ends and a user sends a request to server, the yii forces him to logout from system. I want to know that which part of yii is responsible for logging out such users. As I got the CWebUser is responsible for it but I want to know the exact place (function I guess) of forcing user logout. I want to record a log from user force logouts just before expiring their session. I’m ok with user logout by himself.