Alarm when reconnecting battery terminals?

I have a mid naughties Ford Transit Connect and as per the title this happens. The alarm is not ongoing; it just makes 3 "pamp pamp pamp" shrills and then stops.

To stop battery drainage I want to add an isolator switch at the terminal but don't want to be bothering people with this alarm going off all the time.

I looked in the fuses under the bonnet today, the little diagram on the fuse box lid, and didn't see an image which would indicate the alarm for that, although maybe it is the couple of trumpet icons.

I am in a very quiet area so don't want to be setting it off while I trial and error to find where it is. Might have to drive somewhere else where it would be drowned out.

I am not sure if it is going off in error or it is meant to do that as an anti-theft thing if the battery was tampered with. Either way I want to disconnect it. I really hope it is not in the glovebox fuse compartment as, not thinking of it when making, I built storage shelves there, effectively sealing off access behind them, and have ripped out the seat so would be extremely faffy to get to those fuses. I am barely able to get into the glovebox, let alone behind it!

Btw I have just been reading through my Haynes manual and could not find anything in the fuses section which would apply. The only section I see is the anti-theft system and sadly the only advice they give there is 'go to a dealer' or something to that effect. So would I be stuck with the alarm in that case?