Help! My Volvo is going crazy!

I own a '91 940 turbo stationwagon. She's in great condition and we've kept her very well maintained. We drive her at least 10 miles every day and do quite a bit of highway driving as well. She has 240k mi on her, if that's any indication.

The day before yesterday, my husband tried to start the car in mid-afternoon (not cold), and the engine wouldn't catch. He said it sounded like a record being played backwards. After a while it would only "click." He only tried it a few times, so I don't think the he drained the battery trying to start the thing.

I tried starting the car the next day to no avail. I then tried to jump it (with our '87 740 GLE), but it did nothing. I had my battery and alternator replaced 8 months ago when the same thing happened, so I didn't think my battery was bad.

I know we didn't leave on any appliances, nor were any lights left on (not that Volvo lets you do that).

I ended up having it towed to our nearest Volvo mechanic. He gave me a call today and said that after recharging the battery, it started up just fine. He doesn't know what the problem is either.

What is going on here? I think this is very likely to happen again. What is making the battery drain? Is the battery the problem?

If anyone has any suggestions for my mechanic and me, I'd appreciate it!