That car is chock full of different modules that all talk to each other on a couple of data bus lines. They don't all shut down when the ignition is turned off. After a short bit though they are all supposed to "sleep" once their "after run" functions are done. However when there is a problem sometimes a module will fail in such a way that it puts noise on the data bus lines. This noise will keep other modules on the data bus "awake" draining the battery the same as if you had left the key on in a conventional car.

This is a very difficult scenario to diagnose without detailed information and specialized tools.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff