I agree with most of the point, but car boot will not provide sufficient RF shielding. An opening of just a few cm is enought for RF leakage. The signal can propagate through back seat, windscreen, speaker area, etc and get detected by a receiver.

With all due respect, I think the entire thing is simply bulls**t. When a laptop is off, it is off. Have u ever seen ur microwave turn itself on and heat the air inside by itself?

Metal detector? Your car itself is a huge pile of metal and of different kinds inside and outside, the detector would be ringing like a fire alarm the moment it was turned on.
Wave frequencies? Specifically your WiFi 802.11a/b/g? When it's off, it's off. These frequencies are alot more than your simple AM/FM radios mind you. HOWEVER,there is a single mistake in Ij0000's post regarding the part that car boots shield your waves, no it does not shield your wave. Explain how people can hijack/ride on your house Wireless connection in their car? If these waves can be received from inside your car, the same applies for people outside your car meaning they can also detect your waves emitting from inside your car if you're using Wireless no matter if you're in the driver's seat or the boot. So yes, if you leave your laptop on in the car, it is possible to detect waves, though with a gadget as cheap as rm40 is quite impossible, maybe with a gadget costing in the hundreds to thousands.
I think that some sections of the public are just making it up, but NOT with bad intentions. They ultimate message is just to practice caution, and they just use a story of a high tech gadget to make ppl more aware. Inaccurate warning though with good intentions I'd say.