iPhone 4s irregularly asks for Apple ID after a night at airplane mode
Does not answer question directly as this is possibly a bug with flight mode. Her use case is not what flight mode was made for though?  Better option to keep from being disturbed at night is to Use the Do not Disturb feature form settings.   Configurable and can set it to do automatically during certain times.  Otherwise just manually put the phone in mute mode.

On the flight mode...  I know certain older version of IOS had problems with alarms.   Think it is quite possible that the lack of any network connectivity might block some background processes so can think that it would be reasonable to expect the password at some times.   If it stops n alarm from activating it would be a bug in iOS though.