My 97 P38 just started cranking over in the driveway with no one in it nor any key nearby. I removed relay 16 and it stopped. I tried a different relay and it immediately started cranking again. With the relay removed, I inserted the key and cycled it to start a couple of times, put the relay back in and it seems to be back to normal.
There’s not much to this system per the wiring diagram, relay to BeCM to ignition switch to ground. It’s hard to imagine a switch failing closed, but maybe it’s possible(?). I suppose the relay could be finding another ground somewhere in the wiring, but then why would cycling the switch cure it? And I really hope the BeCM isn’t finding a ground internally.
Have any of you experienced this before or have any other ideas?

I've heard rumours that this can be caused by a failing BeCM but, as you say, if that was the case cycling the ignition switch wouldn't cure it. If the BeCM has been drowned somehow, a high resistance ground on the ignition switch input could cause it, not been wading by any chance? Other than that, it could be a broken ignition switch so there is something that has broken off and is floating around inside the switch.

If you unplug C257 from the BeCM and it still does it, then it is looking towards the BeCM, if not, the switch.

Thanks as usual for the insight Richard. I’ll see what happens next time it fails.

As far as wading goes, it hasn’t been wet since I’ve had it (4 1/2 years), and before me it was certainly never off-roaded. It did live it’s first 20 years in Vancouver, where it is very wet for a third of the year.

i have had ignition switches stick on and you could not turn off the starter, it was the piece on the end of the ignition barrel, don't know if its the same on p38 tho

Thanks mad-as. I was no where near the car when it happened, and it turned off normally when I had shut it down the previous day. I don’t think I’ll figure it out until it happens again and I can do more trouble shooting. In the meantime I plan to spray the switch with contact cleaner, and I think disconnect and clean the 2 plugs at the BeCM.

I cleaned the connectors as noted above, and I replaced the Ignition Switch with a used one I had. It seems to be fine now. If the issue returns it’s something other than the switch. If it comes back it’s either a wire/connector finding a ground, or a BeCM internal fault. Hopefully the problem is solved.

Hopefully that will cure it, otherwise one day it will do it when you aren't around and the first you'll know is when you come back to a car and find a burnt out starter motor and a flat battery.......

That’s exactly my worry, and maybe when I’m way off-grid with no-one else around. Hopefully it’s good now.