The TV receiver is a passive device, right.
No, not even a little bit.
It does not intentionally transmit signals. It is however filled with RF circuits designed to decode the incoming signal, the laws of physics ensure that these circuits also produce signals of their own. Ideally, these are shielded. Realistically, the shields aren't that useful and the sidebands that are being produced aren't being used by other nearby things that will be overwhelmed by the extra RF output of the TV.
FCC laws prohibit TVs from transmitting ... but the laws of nature still seem to overrule the FCC.
This applies to TV tuners as well as TiVo, you're car radio, cell phone or anything else that receives signals. Theres an oscillator in it thats helping to decode the signals, and at the same time, transmitting some things as a side effect. Even your computer monitor is transmitting signals and thats why you can actually read data from monitors you can't even see with the right equipment.
Doesn't work over very much range, but its pretty simple to detect and decode within short ranges (for varying definitions of short range)