Saab 2003 9-5 loses battery power

The battery light (but not the ! warning light) suddenly started flashing a week ago, and then a day later the car wouldn’t start so I took it in for repair. Was told the battery needed to be replaced, but that it was drained because the alternator was defective. So both replaced. Within one day of driving it home, the battery and various other lights started flashing anew (which had happened last after it was jumped and while I was driving it into the mechanic the first time). Car then went dead again after turning off ignition. Wouldn’t start.
What’s wrong with the car? BTW, mechanic has car (which we towed in) and now can’t get problem to recur despite his having driven it around several times, for shorter and longer trips! My sixth sense tells me as soon as I pick the car up and drive home, I’ll get the loss of battery/power experience and it going dead yet again! He says he checked for short wires and belt issues.

You might consider inspecting and cleaning the positive battery cable junction terminal. This is not the same thing as the end that’s connected to the battery post; just close to it.

I can only think that there was a bad connection somewhere with the car wiring for the charging circuit and while the mechanic was working on it the bad connection was fixed, at least for the time being.

Maybe you have an intermittent short, Some things to check, wiring through the driver door, any lights including brakes stay on after the car is shut off? That would be brake, glove box trunk etc, or even radio?