The passenger seats of certain cars use electric field sensing in the seatback to determine if there is an adult passenger present, to decide whether to enable the passenger-side airbag. (Though simply sensing by weight is also common.)

For an example of an electric field sensor, check out the Motorola MC33794.

I imagine people smarter or with more time than me could imagine ways of turning a theremin into such a device.

https://en.wikipedia.org/wiki/Theremin

Not quite the same mechanism, but how about stud finders or voltage detectors?