I Hope you are not using SEP 11.0.X. If yes,
This problem has been identified and it has been fixed in RU7 MP1
Computer status log shows virus definitions as "none"
Fix ID: 2023152
Symptom: When the client starts, SMC sends virus definition date information as "0" to the Symantec Endpoint Protection Manager. The computer status log in Symantec Endpoint Protection Manager shows a virus definition date as "none." The clients have the correct definitions. A workaround is to manually "Update Policy" or wait until the next heartbeat.
Solution: SMC was modified to send the correct virus definition data to the server on startup.
This can cause the machine to generate an alert, however it would be fixed in next heartbeat interval.
Also check this
Virus definitions out-of-date notification is not accurate
Fix ID: 1998671
Symptom: The Virus definitions out-of-date notification does not correctly reflect the filter condition "include only clients which are currently online." This causes more clients than expected to display.
Solution: The client time stamp in the database was updated when a client is offline. In addition, replication of this state has been improved between servers.