Many people have had sat-navs and laptops stolen even when hidden in the boot or glove compartment. How did the thieves know they were there?

If the laptop is switched on it can be detected by its radio emissions, even if it is hibernating or in some form of sleep mode.

Generally sat-navs are computer-controlled and always drawing power - even when turned off. This is because some of the functions need to be permanently powered on. As a consequence, there is a detectable emission from them at all times, detectable when they are put in a glove compartment or the car's boot. Ditto laptop computers.

This makes it easy for criminals to find them using a suitable radio receiver to detect the emissions from the computer, telling the criminals that there is a device in the car and even where it is; boot or glove compartment.

But if you wrap the device in baking foil it acts as a Faraday's cage and screens out all the emissions.