Battery is being drained or just old?

[Jason Beal]

My battery (240d 1981) was weak so I took it in to be tested. It tested as a good battery, but needed to be charged. When I came to pick it up, the guy was laughing saying that it was driving him crazy because it just took forever to hold a charge. Then we placed a load on it to see how it held out. It was fine. I took it home put it back in the car and it fired right up. Two days later it was dead again. Could there be anything draining this battery on my 240d? The engine stays running when I turn it off and I haven't addressed that yet, but I thought that was vacuum related. Or does it mean it's just time for a new battery and it's on it's last legs?

Thanks for input.