The sensors in your wheels do emit an rf signal, they each emit a unique identifier, or code if you will, to equipment up to 50 feet away on a frequency on 314.9, 315 and 433.92 MHz.
The ecu is capable of transmitting a "wake up signal" to the tpms sensors on 125 KHz, then waits to hear from the tpms sensors to transmit their id's. It then checks it against the code that is programmed into the ecu.
The tpms sensors can also (some, not all, not sure if ours is the "enhanced" tpms sensors or not) transmit a signal to the ecu to warn of a quickly deflating tire before it even gets to the programmed low pressure psi.
And although I'm unsure of this either, but I think those with an alloy rim for a spare has the 5th tpms sensor installed, but not the steel wheel. Perhaps someone who has a limited can verify this (I know the limited came with alloys, for some reason they were painted yellow!).