Bonnet open sensor is a popular one.
You can put the alarm into test mode by opening and closing the door 8 times (I think 8) in quick succession. Alternatively, press the bonnet sensor (a plunger on the drivers side of slam panel) 8 times. But if the sensor is faulty the alarm test mode wont kick in.
With the alarm in test mode the alarm should beep and/or the lights flash each time you open a door, bonnet or the boot. If it doesn't then a faulty sensor. I had to replace a couple of sensors on both of my Mondeos. The older Mondeos are a pain in the rear as the sensor is on the back of the lock inside the door! Not easy to get to!