1997 Honda Civic battery dies overnight

Electrical problem
1997 Honda Civic Front Wheel Drive Manual 100000 miles

Hi - My 1997 manual honda civic battery is dying when it's parked overnight. The mechanic charge-tested the battery and says it's in good shape (over 70%) and it's a newer batter - about 1.5 years old. The alternator is also okay, he says, because the battery charges while I'm driving the car - just dies when parked. About 1.5 years ago I got a new battery when the electrical system failed - while driving, all my radio and lights went off and a few seconds later the car stopped and the battery had died. My mechanic at the time identified a faulty O2 sensor and blown fuse and I got a new sensor, new battery and new alternator. This last winter I stored the car in an unheated garage over the winter for 5 months and did not start it regularly but when I started it 3 weeks ago it went fine and I've been driving it regularly for 3 weeks for long drives with no problems. Then the other morning I went to start it and the battery had died. This time the electrical system (radio etc) still works and it doesn't seem like the same problem as before, but there is still something going wrong w/ the electrical system somewhere, according to my mechanic. He says there is some kind of parasitic drain on the system that's causing the battery to die overnight but that it might take a long time to detect the exact circuits that are causing the problem. Do you have any experience with this or suggestions where to start?