Make sure nothing has been jammed under the boot handle to stop it locking properly,i know you said you checked the door locks and windows but did you check the boot lock?

 

What does it even sound like? I've never seen it go off, how do you even make it go off if you want to?

 

I know it's not really a fix but have you tried pressing the alarm button on the ceiling (wifi looking button) before getting out and locking the car? This disables the interior sensors of the alarm.

It may stop your false alarms till it gets sorted properly

 

Fundamentally at least, the alarm works with ultrasonic distance meters which are fairly reliable in enclosed spaces. I made a unit a while back using those that detects people walking up to our door, even outdoors it worked well.
If the alarm is programmed correctly to discard single false detections then your source may be external. It works like a bat, bounces ultrasonic waves off the car and "times" the return signal, as simple as that. If something gets in the way it reduces the return time - or another thing that can happen is a false return signal. Which could be coming from anywhere, passing traffic being a common source.
Obviously check the basics like do you have car seat covers that might be confusing it (a total stab in the dark), or anything left on the dashboard. If it's only happening when plugged in it does imply maybe the charge circuit is interfering but one or two incidents isn't enough to fully correlate - I'd hedge my bets on a faulty ultrasonic sensor being the cause - they're up in the ceiling and I've noticed sometimes condensation forms up there at least on the lights.

 

The Ampera will sound the alarm when the charging cable is disconnected, and a momentary loss of communication could cause the alarm to think the cable has been removed. However, I'm sure the Leaf doesn't have this, as we normally unplug ours before unlocking the car (although I do have the key nearby).

If you can get a garage to plug in some diagnostic equipment, the alarm system normally records the last few causes of activation. This will narrow the problem down considerably.

 

I know it's not really a fix but have you tried pressing the alarm button on the ceiling (wifi looking button) before getting out and locking the car? This disables the interior sensors of the alarm.

It may stop your false alarms till it gets sorted properly

Well worth a try - you never know you may have a spider in the cabin that crawls over the sensor from time to time.

Have you checked the 12v battery voltage at all ?