Notebook Battery Drainage

I have a HP Pavilion zd8000 which is run only off the AC adaptor. It still has the original battery from '05. Occasionally it is left with a relative for two or three days, for safe-keeping, not usage, with the power turned off. When it is returned an icon indicates the battery is discharged to the 87 or 89% level, and stays on until it is recharged.

Is there any way this happens other than by breach of trust?

 

Batteries naturally discharge over time. That battery is probably near the end of its lifespan.

http://h20239.www2.hp.com/techcenter/battery/battery_ts.htm