Most receivers also broadcast at a very low power. They aren't intended to, but nothing is done to prevent it, so they do. It should be quite possible to build a TV that the 'detection vans' couldn't detect. Industry doesn't bother, though.
What do receivers generally broadcast and why? (I'm going to use TV for this example)
1) You need to know a bit about electronic mixers. A mixer is a device for combining two (or more) A/C signals (generally Radio Freqency). Lets say you put signal A (@10Mhz) and signal B (@1000Mhz) into a mixer. At the output, you get 4 signals, the sum of A and B frequencies (1010Mhz), the diference (990Mhz) and the two originals. You'll see why this is important in a bit.
2) The information for a TV broadcast is first encoded on a RF signal of about 10 or 11 Mhz. This is called the I.F. or Intermediate Frequency. TV broadcast signals all start with the same IF freqency.
3) The final broadcast freqency is obtained by mixing the IF with a signal that is 10 Mhz or so (whatever the IF freqency is) shy of the final broadcast frequency. For example if you want a final broadcast frequency of 1050Mhz, you would mix the 10Mhz I.F. with a 1040Mhz signal and filter everything but the 'sum'
4) When a TV recieves a broadcast, it is taken back a part to get the original 10 Mhz I.F. signal.
5) So, the TV makes a 1040Mhz signal to mix with the broadcast so that it can get the 10Mhz I.F. out of its internal mixer.
2 things frequently leak from TVs and are broadcast by its antenna, the 1040Mhz mix signal, and the 10Mhz I.F. These are detectable from outside the house. Mostly the mix signal is what gets out. So, the 'detection vans' listen for a unmodulated signal 10Mhz or so under the known broadcast channels.
The recieving path of TV can be made to not leak those signal back out. It just takes an extra filter or two. But, since it would not improve the function of the TV for what it is bought for (to watch TV), the expense is rarely or ever justifiable to the TV manufacturers.
