While cruising through the SAP Security Guide one day (a great reference for security and ITS connectivity how to by the way), I came across the system parameter transport/tp_logging. Default value is "ON" but more informative is the table name TPLOG in which TP actions are logged. Using SE16 and specifying this table, a nice selection criteria screen is displayed so that you can zero in on TP actions performed by a specific user for a specific time frame for example. Great for a quick way to look at who transported what when in cases where something questionable got moved/transported.
SE16 on table TPLOG