There are new 60ghz sensors available that can do this (they can see through walls so you could have the sensor completely enclosed (maybe even potted in epoxy?

There are new 60ghz sensors available that can do this (they can see through walls so you could have the sensor completely enclosed (maybe even potted in epoxy?!)). Sparkfun/Acconeer A121/XM125 is what I'm using,although not in this context, it's for my robot.