I recently got an iPhone 4 (with iOS5). I almost never have the ringer on, so the phone is mute and alerts (like incoming calls) come with a buzz. Periodically, while the phone is idle and no call in coming, the phone buzzes. I figure this results from some notification, however no recent notification shows up in the notification finder, nor have I recently received any emails.
How can I figure out what is causing these alerts?