That looks like a Viper(DEI) Proximity Sensor that hooks into an alarm. As said above, movement inside the vehicle(as well as outside), will trigger the alarm. When set properly, motion within a certain distance from the outside will sound a warning alarm, and if you go inside the set distance it will go off. Very common to use in a convertible. Follow the wire, and you should find the actual alarm, unless the PO removed it.