1997 Ford Taurus Passive factory alarm system

I've owned this black colored LX wagon since new and take immaculate care of it. Other than a rebuilt Tranny at 103k and a rebuild of rusted out rocker panels last year, the car looks and operates almost brand new.
My question is this:
When locked, the alarm system will sound the horn without provocation at any time. I've tested this at my house and am sure of it. I've queried my mechanic and he has no answer for it. Now I cant lock the car unless I diisconnect the horn and am fearful of a non-break in. Please help!

PS: The doors and hatch close correctly and the automatic door locks work as well.