As car security technology increases, cunning thieves are turning to more and more imaginative ways to steal them. Personally I wasn't aware that they were creeping into unlocked cars at petrol stations and momentarily left outside shops, then leaping up as you drive off and threatening the keys from you, but it's obviously a scenario Volvo were concerned enough about to build and unauthorised-presence detector into their S80 executive flagship.
Using a sensitive heart-rate detector, stowaways in your Volvo are identified and the keys (which hopefully you've taken with you) alert you to the fact that something is amiss. Less frightening (but perhaps more useful day to day), the Personal Car Communicator (PPC) can inform you whether or not you've locked the car.
It's available now on the S80 model, though I would imagine that eventually it'll be extended further down through the ranges.