Recording user logins and logouts

Hi, I have recently installed LOGman and have noticed that some users log-in and log-out and both actions are recorded, and at other times users log-in (which is recorded) but then just disappear - in the sense that they are no longer logged-in but also not recorded as logging-out. Could this be because they may have instead been timed-out of the session, which means they were not recorded as actually logging-out, or is there some other reason?