The alarm triggering for no apparent reason is normally one of a few things. Have you tried using the key itself to unlock and lock the door, sometimes this resets the alarm system if the two are tied together (they normally are). The next to try is changing the fuse that controls the alarm which could be faulty, change the fuse that applies, this should be obvious from the state of the fuse as it will either be broken or burnt. The fuse box is normally situated on or under the dashboard somewhere and should be shown in the car manual. You should also find a diagram either in the fuse box or in the manual telling which fuse to check for the alarm - if not ring up a friendly Ford dealer and ask them which number fuse it is.
The second possibility is that you have water leaking into the floor of the car, this is where the wiring is normally located for the alarm system, if this gets wet, it corrodes the wiring and the alarm is triggered as it is considered to be a tampering with the wiring. If this is the case, you can take it to a garage and they can fix the problem, but be prepared for a large bill!
Good luck!