There might be a low current short circuit.  Short is not big enough to
blow the breaker/fuse but enough to drain the battery.

The other possible reason is that the battery itself is bad.  Did the
battery go though a deep discharge?  Like something left on 24 hours
and the battery was dead.  Recharging these batteries sometimes work.
But deep discharge batteries go though an atrophy (destruction) and no
amount of recharging will resurrect them

gamini

It depends... How old is your battery? Last I remember... ML430 is last
produced 2001... so that means your battery could be 6 years old.

Modern cars use alot of standby electricity draw... so with a old battery
that is on the way out... it could drain it in 4 days.