There is a log message which the ASA generates when a user session terminates. It gives a time stamp and a reason (and some other information).
Here is an example of this message:
Jul 27 2010 13:45:41: %ASA-4-113019: Group = test, Username = test_user, IP = 1.1.1.1, Session disconnected. Session Type: SSL, Duration: 1h:18m:16s, Bytes xmt: 41764496, Bytes rcv: 4131569, Reason: User Requested
HTH
Rick
HTH
Rick