1988 245DL / AW70, 290k miles

Daughter's car; all of a sudden the battery charge has run completely down. (She says she left no interior or exterior lights on).

Where would I begin to troubleshhot for "parasitic" battery drain?

After jump starting, the alternator charges ok. I say this because the car runs while all accessories & lights are on and with the battery completely drained. While running, the dash voltmeter needle indicates in the middle of dial.

Battery is an Interstate, about one year old. The shop said the battery was so low, they had to charge it before it could be tested. Once charged, the battery tests out Ok.

I am surmising that there is an electrical battery drain somewhere. Advice?

Thanks