excellent ,,, will check ignition switch carefully,, thanks Doug
if that checks OK not problem,,, any other advice ?? ,,, no warning lights on dash (door ajar etc).
If the ignition switch does not appear to be the problem, I suggest back tracking from probing the BCM fuse for voltage drop - I'm not sure how accurate that it - at least, I've never used that technique.
What I've done is to connect an ammeter in series with the battery, and then use a removable jumper across the ammeter.
Most ammeters cannot handle more than 10 amps, so you cannot crank or run the car using one, but you can measure the current AFTER the key is turned to OFF. (BTW, jumper wires cannot usually handle crank current, either.)
With the jumper in place across the ammeter, turn the key to ON, wait a few secs, then turn it to OFF, wait a few seconds, then remove the jumper from the ammeter and watch the current for a few minutes and see what it does.
Ultimately, you want to see the current go down to the 10 or 15mA range. If it stays well above that, that confirms the parasitic draw. At that point, I would begin removing fuses from the fuse boxes one at a time, and see which fuse drops the current down.
Depending on what happens when a fuse is removed and replaced, you may then need to re-connect the jumper and cycle the key back ON and OFF to get the system back in the same state it was before you removed the previous fuse. It's important that the system be in the same state each time the next fuse is removed in order to be sure the parasitic draw is present.
As you can see, it can be a painfully slow and tedious process. In your case, since you already suspect the BCM, I would start with that fuse and fuse box.
After that, if it is indeed the BCM, the next step would be to get a drawing with the all the inputs to the BCM and to probe them one at a time looking for one that's not correct, and that can be difficult. Furthermore, it may not be readily apparent what the proper level should be. The other thing to keep in mind is that, when probing signals, you need to have a high impedance probe such as a typical voltmeter (DMM) has. Otherwise, such as with a probe light, you can load the signal thereby affecting its level thus getting an inaccurate reading. That could also cause the BCM to see a different level causing its behavior to change.
Another thought, there are four big fuses (as I recall) just downstream of the battery which feed the various fuse boxes in the car. By pulling those (one at a time), you can isolate which fuse box has the parasitic draw (in case it's not the one feeding the BCM). That is, you want to be sure you have the right fuse box isolated, then isolate which fuse in that box is the culprit.
All that said, if I had to roll dice on this, with the parasitic draw established, I'd pull the On-Star fuse first.
Anyway, those are some thoughts I have on it. HTH.