VE Omega wiring question

Hi Guys, first post, my names Steve.
My son has a VE2 ex cop car which we need some help with and were hoping you guys are the right people to ask.

The car came with a dual battery setup in the boot which I'm assuming the passenger side battery runs the car and the drivers side is for the police accessories.
He lost his licence a few months ago and as it just sat for that period the battery went flat.
I removed both batteries thinking they would both need charging, however, despite the battery on the passenger side being dead flat, the battery on the drivers side was still fully charged.
With this in mind we discussed removing the secondary battery altogether as it obviously isn't necessary to run the car and takes up space and is considerably heavy to be lugging around for no purpose.
So I removed the extra battery mount and removed all the wiring where it had been added to the existing battery, however, I was left with one wire that I'm not sure what to do with.
From what I can tell it comes from the wiring loom on the passenger side, underneath the rear parcel shelf, is brown with a blue stripe in colour and had been extended to reach across to a redarc gizmo attached to the secondary batteries setup.
The question is, what is this wire for and do I need to reattach it to the cars primary battery somehow or can it be insulated and taped up out of the way.

Thanks

 

Thanks for the reply
It goes into the circuitboard under the clearish cover attached to the side of an isolater like this one

https://www.sparesbox.com.au/redarc-battery-isolator-24v-100a-max-sbi24-sbi24?utm_source=bing&utm_medium=cpc&utm_campaign=SPX - Shopping - Sub Category&utm_term=4579328490979548&utm_content=ALL Products

I hope this makes sense
The blue wire on the far side, what should I do with the wire which connects to it that feeds back into the cars wiring loom if not running the second battery, id like to make sure its not going to cause any grief before I reinstall the primary battery.

 

An override feature apparently, so the secondary battery can be momentarily activated to power the car.
I just taped it up and secured it up under the lining and reinstalled the primary battery. The alarm sounded so I locked and unlocked it with the remote and it was all good.
Then when I started it there was no power to the dash and display and I thought i'd fried it but turns out the Police stealth mode thing had activated, so I tapped the switch and presto it was all there again..lol

Bonus is we now have a complete dual battery system with 70ah battery for camping etc.

Thanks again.