Found a factory car alarm in my 190D?

[dsvw]

I have a 1986 190D

I returned to my car after it being parked for a couple hours today and the horn was blasting at an even, steady interval.

I didn't realize it was my car at first, and when I put the key in the lock to open the door, it stopped immediately.

I am unable to recreate the alarm. I tried to set it off and can't get it to happen. I do see what looks like a horn and some additional wiring in front of my battery, so I think I found the unit. It appears to be factory.

I don't see anything in the interior of my car or anywhere else that would enable or disable a car alarm.

Did these cars come from the factory this way? If so, any idea how to turn it on and off?
Thanks.