Oracle auditing on specific user activity
Thanks to Andrew Brennan, I created a trigger that enables tracing when the user logs on:

CREATE OR REPLACE TRIGGER USER_TRACE_TRG
AFTER LOGON ON DATABASE
BEGIN
    IF USER = 'SCOTT'
  THEN
    execute immediate 'alter session set events ''10046 trace name context forever, level 1''';
  END IF;
EXCEPTION
WHEN OTHERS THEN
NULL;
END;
/