TV detection

Hi,
sneaky question, I am an electronics newby, how do TV detection vans detect one of these chips in use with live tv in a pc? Or is it more to do with detecting an aerial that is in use?
Thanks guys!
sneaky question, I am an electronics newby, how do TV detection vans detect one of these chips in use with live tv in a pc? Or is it more to do with detecting an aerial that is in use?
Thanks guys!