You enables the option "Display notifications about detections when the user logs on". Symantec Endpoint Protection detects a threat but a popup message dialog of the detection does not display after a user logs on.
SEP 14.0 MP2
Operating System wasn't ready when the alert dialog was displayed.
This issue is fixed in Symantec Endpoint Protection 14.2. For information on how to obtain the latest build of Symantec Endpoint Protection, see Download the latest version of Symantec Endpoint Protection.