Thanks Gatos. I haven't owned the car long and the last owners will be of no help, as they were young Finnish junkies looking to flip the car for dope. The brackets and wiring looked stock on this but I guess it's possible that this is part of an aftermarket system, which would mean there is a separate box somewhere else that contains the (aftermarket) battery in it, and this horn I extracted was just wired into that. I'm hesitant to remove things to start tracing the wires from that horn, but it seems the only way to find out. All I know for sure is that there is indeed an alarm battery somewhere. The alarm was/is still triggered despite removing the main battery, disconnecting the central locking, etc.

Any other tips on where to look appreciated!

Can you post a photo of the connector? Might be able to tell if it is genuine or not. Do the wires become part of the main loom or are they on the own? If I was to guess, something like this would be fitted inside the car and most probably behind the dashboard, on either the drivers or passenger's footwell.

Are you by any chance at the Facebook Volvo S40 V40 classic group here: https://www.facebook.com/groups/972416682830534/ ? There are quite a few people there, with Phase 1 models that might be able to shed some light too.

I will get a photo of the connector soon, but you can get an idea from the black receiving end of it located on the horn in one of the photos. It didn't look like the kind of clip I could purchase, rather it had some shaping to it. The wires went into a larger loom intersecting from around the area, and the steel bracket itself had some sort of recess to where it attached to the frame. Seen together under the years of matching corrosion, had a stock appearance. I'll check the group, thanks!

https://ibb.co/bPkBcGj