I have a 96 850 and the hood theft sensor is on the front left hood latch(right side if you are facing the car). There are three latches for the hood. One on each side and the center one. All are released by the interior hood release beneath the steering wheel. On mine only the one on the front left has and additional electrical connection that is tripped by the hood latch engaging. This send info to the alarm system. If the hood is opened forcefully from the outside while the alarm is armed the the theft horn blows and the four way flashers activate. The ignition may also be disabled. I have my hood latch disabled electrically because of other problems with the theft system. I have seen several of the theft alarm modules on ebay. Fairly easy to check the part number by removing the cover under the steering wheel and twisting yourself into a knot then looking up into the area close to the footwell of the dash.
You might hear the relay clicking and see the led in the windshield are flashing if the module is acting up.
good luck