Lexus: Dead Battery Syndrome

I have a 1999 ES300 with 88K miles.
The battery has been going dead about every 3 months for 9 months now.
I am wondering if the alternator has the voltage regulator built into it.
Apparently the problem must be in the alternator or there is something
draining the battery that is a slow drain and takes 3 months to flatten the
battery.
Jump the battery and it starts and charges and then starts just fine so the
alternator must be putting out.
I'm just looking for suggestions. I don't have any lights that are stuck on
and I checked the trunk too.
Any ideas?

Bugsy