Imagine this..
MIlitary defense have invested a lot in building robust and reliable radar systems. The research that has gone into that is not only the building of electronics, but also design of antennas and field measurements on various objects or antenna reflections (e.g. structural vs electrical scattering properties).
Backscatter is used in RFID, which can be used anywhere from strict access regulation (e.g. to police or military stations), or for an electronic door lock in a public/large building.
Going 1 step further, we want to look into using backscatter radio for building wireless sensor networks. Since creating reflections is such a cheap way of broadcasting data, it severely cuts down on battery consumption and thus operational costs.
RFID, much like radar, requires an excitation source (a sine wave) to create reflections from. However, that requires infrastructure which could be a hassle. So recycling existing RF signals is being investigated with ambient backscatter radio, which has been successful for various signal sources (TV, FM, WiFi, Bluetooth stations).
Now imagine a world where we can have sensor devices everywhere, and nobody needs to go around replacing batteries to keep them on, and these sensors can piggyback their communications of existing signal powers. Imagine a government information agency or the military that can develop a device which continuously transmits mic data from a room, using existing RF infrastructure, permanently integrated into e.g. furniture or building structures to keep them hidden.
Wouldn't that be an awesome spy/intel device for military/gov? Is that sci-fi enough? Probably. But this story has gone full circle, and I imagine virtually any tech can do that. Now personally I work on the IoT part of it.. but looking back and forward you can always identify military applications. Even then any tech that has a great value in military can also benefit civilians: always-on sensors are an example, but also imagine better radar systems for airports and air space control. Or future warehouses if we replace all Amazon employees with drones to fly and navigate them indoors (which if reports are correct, are running out of people since high turnover rate).
The problem is probably that when people are going to mount real weapons on those drones, and navigate them radars or GPS, that now drone, radar and GPS technology have all been militarized and are bad. I don't think that's the case. A water cannon can be used to split riot groups, extinguish a building on fire, or be used for events to give the crowd a blast (literally).