It is the tone that I had selected as the "Default notification sound under Settings/Sound and notification/Ringtones and sounds/Default notification sound, which is the Postman bell sound. I use a different ringtone for calls and calendar, and have it turned off for email notifications, as well as Facebook notifications.
At random times throughout the day, the Postman ringtone rings, but nothing shows up on the notification bar. Phone is charged, and on the times that I did get the "charge required" notification, I could see that in the notification bar.