Didn't they have seismic sensors? It would work best for motion detection via distance triggers, maybe in conjuction with "emmission" scanner similar to the one from Contact DLC, available in FOB/HQ. The most suitable objects simulating this type of sensor could be some of AP mines, maybe? The pros of this approach is you already have an animation for deploying the "sensor" and the model of it. The cons would be the need of creating a config patches for the sensor and its monitoring device.
Nice idea, by the way!