Battery goes completely dead in 2017 Expy overnight once a while

[99WhiteC5Coupe]

I was a mechanic and normally fix things and might still after I eventually get a new battery. This one is 5 years old but very strong. The problem only happens about every month.

What could be wrong with a five-year old battery?

 

[17expi]

What could be wrong with a five-year old battery?

It works very well other than this parasite problem.