2002 Focus "door ajar" light won't go off...

Can anyone tell me why the "Door Ajar" light would stay on even though all doors & trunk are closed tight? We think it has to do with the trunk, as we only just got the car and the only door that had been used was the driver's door and everything worked fine. Then my daughter went to the grocery store & used the trunk, and the light now won't go off sometimes. There have been two occasions when pressing the trunk button in the car made the light go off, but most of the time, it just stays on. Thanks!