Ideas about why the battery is running down at all, when the phone is switched off. For example;

User uses phone for 1 day while they are on business. Battery life ~90%
User turns the phone off completely (I mean actually press and hold the power button to shut it down)
So many weeks pass when they need it again, so assume that as they turned it off with 90% battery they should be able to just power it back on
Realise this doesn't work so need to charge it