You can get cheap microwave motion sensors from ebay the kind of thing meant for outdoor security lighting, one of these angled correctly might give you the correct range you want.
But it would also pick up people.
To do exactly as you want you'd need some kind of modded metal detector circuit, I've never seen one for that purpose.
Back in the 90's you could get car alarms that talked & made silly noises, they sometimes had proximity sensors on them, you might be able to get the sensors separately still.