Some windows smartphones will sound all the alarms that shold have gone of during the period it was shut down (outlook on the PC does the same) may be its some thing you have set to go off through the night but forgot about...you could leave it on for 24 hours on loud profile and then when alerted you'll know what caused it...horrible thought I know but that could be your only solution if you haven't figured out what is by any other means ( check alarms and check calendar appointments) also some people running wm6 report that alarms still sound on the default alarm even when the have deleted it.
I really should use punctuation to better effect!