This issue is either old battery (anything over 2 years is old) or the owner has it set to SLEEP rather than shutdown and the battery drains when the owner thought it was off.

Be sure the POWER SETTINGS are to power down when you press the power button. For my laptop I have it set to SLEEP when I close the lid for fast wakeups and then POWER DOWN when I press the power button. All this is under your control in almost every OS.