How old is the battery? The OEM Volvo batteries last about
3 to 4 years. Yours could be on it's 2nd battery and may need
a third (98s were sold starting in April 97 so some are about
8 years old now -- mine is).

I'd also check the lights over the rear doors if you have kids
(mine click them on all the time and you don't notice since the
car slowly dims the lights and you have to come back after a few
minutes to check them.) They do get left on over night on occassion
but have never actually run down the battery.

The rear trunk light is another one to check. Just leave
the rear seats folded forward when you lock the car up to
check that one.