Had issue again due to excessive rain last night.
When I came to my car, the alarm went off and then over-rode it with the key.
Had to take the relay out to stop the battery draining.
As it happened, I bought a new car the night before so no problems anymore for me.
Good luck finding the root cause of this problem. (I'm an engineer and I couldn't).
Maybe the car was just being temperamental.
Cheers
Lee