Holden Cruze Immobiliser

[pman92]

Hey all,
Had a holden Cruze 2011 diesel model towed into work yesterday. Battery was flat (because automatic headlights were left on) and wouldn't crank even with jump pack connected.
Checked all fuses and determined the PCM was not energising the starter relay.
Live data on the scan tool displayed the ignition switch was going in 'crank request' position, but the starter relay command remained 'off'. Also noticed 'Immobilser system status' remained as 'Standby' and ECM challenge status remained 'invalid'.
There were a number of DTC's but none returned after being cleared. They included a 'U0140 Lost Comms BCM' and 'P068A engine controls ignition relay de-energised too early' and 'U0418 Invalid data from BSCM'. The others related to AC pressure switch, DPF high temp, water in fuel etc.

Anyway this morning all I did was remove the starter relay and confirm the pin 30 power, 86 earth etc. and jumped 30 and 87 to confirm the starter motor was working. As soon as I done that I put the test light on 85 (PCM signal) and hit crank and to my surprise it lit up. Sure enough plugged the relay back in and it started and hasn't faulted since.
I've noticed now in live data that the start relay command now switches to on as expected. Immobiliser system status switches from standby to 'pre release' as soon as you crank. Very shortly after ECM challenge status switches to valid and immobiliser system status switches to release. ECM challenge status soon switches back to invalid but the car keeps running and immobiliser system status remains as release.

Does anyone know what could of caused this vehicle to become immobilised? I've got a vey rough idea how the immobiliser system works by can anyone elaborate? How come it has fixed it self?
Any information at all appreciated.

Thanks in advance

[pman92]

Also note I had tried locking and unlocking the doors. The security LED on the dash stopped flashing even when it wouldn't crank. And the battery was not disconnected or any fuses removed at any point (they were all checked by back probing). It still failed to crank this morning even after the battery was charged.
It's like it had a moment and didn't work until I cranked the engine over by jumping the relay connector.
I doubt it is an intermittent relay fault as the live data on the scan tool doesn't back that up.

Also note it's actually possible to plug the starter relay in backwards. But this hadn't happened.