I have worked through a number of intermittent battery drains in 240s over the years and if what you have turns out to be an intermittent drain, then I'm willing to place a bet on one of these two VERY common 240 issues.

1. The dome light delay relay. Located under the driver side, clipped to the brake pedal box. Black, cylindrical in shape. Common fault when they get old. Once in a while they hang open after closing the door and will kill a battery in 6 or 8 hours. Solution: simply unplug it. You will still have a dome light. It just won't delay.

2. One person posted a vague reference to the power door locks. On nearly every 240 I've looked at, the wire insulation is flaking off the wires for the PDL actuator and the key lock switch, both in the driver door. This is not exclusive to pre-1988 cars and the problem harnesses. This issue affects EVERY 240, period. When these wires intermittently short, they do things like cause the locks to pull down or push up, and stay that way until the battery drains. The PDLs are active all the time, not just when the key it on. If you ever disconnect the battery and upon re-connecting it, you hear a click coming from one of the doors, this will be it. Solution, open the door panel and repair or re-insulate the rotten wires.
Dave B.