About 10 - 15 seconds after I wake my Mac from sleep and log in with fingerprint, I get the system alert "funk" sound once. There's nothing else to indicate why it plays..
A restart typically doesn't leave much time for a new event like a new message to happen. Sleep usually involves more time for something to notify you about to happen.
One easy way to check if it's a notification, at least for those controlled by Notifications, is to turn on Do Not Disturb in Notifications and see if it continues.