Occupancy sensor to turn off socket
You could combine a standard Occupancy Sensor:

with a Relay:

The Occ Sensor controls the relay, and you use the relay's Normally-Closed terminal to power your Outlet, something like this:

So when the Occ Sensor is off, your outlet is on.
You would probably need a 2-gang box to be able to fit the sensor, the relay and all the connections (the relay is mounted outside the box through a knockout, so make sure you get a box with knockouts.