Spotted this thing today. Not sure what it is though

Well clearly that's a tail gunner position, just missing the gun at the moment

The fuck!?!

My guess is a Prius owned by a super-miler. They are a group of people who try to get the best mileage out of their cars by making all sorts of seemingly bizzare modifications like the flat disc wheel covers and an aerodynamic tail section.

Tail gunner on a Prius clearly.

It's just a mileage maniac trying to streamline their Prius.

It's called a boat tail and it helps with hypermiling, a competitive sport to maximise miles per gallon on a car, usually through narrow tires, improved aerodynamics and driver techniques.