It normally turns out to be a hidden alarm - alarms are managed by a custom ASIC (called retu) which may have it's own small area of non-volatile memory or may access an area of internal flash memory.

The aforementioned threads should provide information on how to clear any hidden alarms. If the N800 continues to turn itself on after the alarms have been cleared a bug should be opened in Bugzilla (heck, a bug should be opened anyway if one doesn't already exist - having users hack around to clear hidden alarms is a nonsense...)