So, every once in a while, when my Mac Book Pro is sleeping, I’ll hear a default notification type sound come from the machine.
I’ll look at the mail, check Reminders, Calendar events, and the Notification screen and see nothing on any of them.
Any suggestions on how I can figure out the mystery of why this default sound is being activated periodically?
This would happen if the notification settings for an app is set to "Play sound for notifications" but the rest of the notifications settings are such that you wouldn't see it in Notifications Center.
Go to System Preferences, Notifications. There will be a long list of apps there. One by one go through them. Look for "Play sound for notifications" set to on. When you see that, check the alert style. If it is set to "None" instead or "Banner" or "Alerts" then that's one situation where you could get a sound and nothing else. But also check for things like "Show in Notification Center" to be off as when you are sleeping you'd miss a "Banner" and then there's be no trace of the notification in Notification Center either.
By switching off "Play sound for notifications" for apps where you don't want a sound, you could fix the problem, but maybe never know which app was bothering you. On the other hand, you could switch on "Alerts" for everything as well as "Show in Notification Center" to try to identify it.