There is an ignition control relay under the dash, and an engine control relay under the bonnet.
Perhaps one of these control relays is stuck "ON" and feeding voltage to the computers' 12V battery feed fuse, and the computers are using their "normal" quiescent current.
You may have to monitor the common fuse (under bonnet) to the computers and sequentially remove the relays under the dash and then the engine area. It might turn up a stuck relay.
The other way is to monitor the computers current draw, 3A, and then give each relay in the control circuits a tap with a screwdriver - this may disturb the faulty armature contact and help you out.
Failing that, remove the connectors to each computer module and watch the current if you cannot tie it down to external circuitry.
Is the vehicle giving any screen based alarms or trouble codes? Maybe a TEch2 could help. Something like the rear window demister isn't stuck on, or a back feed problem to the alternator? The BCM can be involved with a lot of the accessories' operations as I see it.