battery drain while parked - why?

["Creel, Kenneth E" <kenneth.e.creel@lmco.com>]

I returned from two weeks vacation to discover this morning that my battery
was completely dead.  (Radio was off, doors closed).  This has happened
before, in only five days time, when I was working on the car.  At the time
I thought I must have left a door ajar and dismissed it.

a) Is there some Audi part that's infamous for causing this problem?  Some
place to start troubleshooting?

b) Any recommendations for a cheap, basic, automatic (i.e., attach and
forget) battery charger?

Thanks,
Ken Creel
'90 90q20V