Once an area has been identified as a potential target, the US military can use a device known as an unattended ground sensor to sit and wait for something to happen. These are designed to cut the "sensor to shooter" time - the often crucial gap between gathering intelligence and sending in military forces.
The most sophisticated sensors can alert military forces by detecting:

Vibrations caused by people or vehicles

Noise including machinery or voices

Night-time movements picked up by infrared

Speed and direction of movement, if the units are in a network
Once alerted, the sensors relay information to a command centre where a laptop may be sufficient to process the data and provide enough information to launch a strike.