My neighbor was commenting that his wife lost his spare key somewhere in their house. It’s one of those newfangled wireless keyfob keyless ignition systems. I jokingly said that all he needs is one of those teenagers who steal cars by emulating the keyfob signal and he could be driving it again in no time. That got us wondering if there really would be a way to use a radio receiver to home in on the fob signal in his house. Presumably, he could tune a radio to the right frequency and gauge the signal strength as he walked through the house. But what frequency? Does the key pulse on its own or only when it receives a signal from the car? Could the key fob be induced to emit the signal to aid location? Would any receiver be sensitive enough to detect the signal strength difference over a range of, let’s say 40’ to two or three feet? If it matters, it’s a 2019 Ford Fusion Hybrid.
If we find out, I have my fingers crossed that the remote I lost somewhere in the house two years ago still has enough battery left to respond.
The fob emits no signal until it “hears” the car signaling. Otherwise the battery in the fob would only last days, not years. The fob signal is real weak, as you can prove by how close you have to stand to the car before it’ll auto-unlock.
You’d also need a specialized receiver to listen on the right frequency for the right sorts of signal. While also fooling the fob into thinking your receiver was not only a car, but the correct car.
Not gonna happen.
I suppose if you did have a cooltastic fob-receiver you could drive the car around inside the house and once you got the car within a couple feet of the fob your receiver would chirp. But that method is real hard on your bric-a-brac. Cannot recommend. 
There are ways to detect even a powered-down antenna (basically, you send out a pulse at the antenna’s resonant frequency and look for something that resonates with it). I don’t know if there’s any easy off-the-shelf option for that, though.
With a bit of sleuthing, I think Ford still uses Texas Instruments DST80 based RFID devices.
These work at 132kHz. Which is a barely RF. Dolphins can hear that high.
As noted above, they don’t transmit until interrogated, and I’m pretty sure they won’t reply unless they receive the right challenge code.
The car thieves attack vector is to use another transceiver to boost the power between your car and the keyfob - basically a repeater. They stand by your car and point an antenna at your house and hope to convince your car that the keyfob is right next to it. Similar tactic would work, if be a significant amount of work, to aid in finding the keyfob.
My car has the amusing habit at night of illuminating the lights under the door mirrors if I walk past. Which is very considerate of it. You don’t really need an active repeater. A long bit of coax, and a couple of loop antennas tuned to 132kHz at each end. One end next to the car, walk about inside sweeping the other end’s antenna about and see when the car lights up thinking the keyfob is near.
How does a pocket-sized device even couple at all to a wavelength that long (a couple of kilometers, unless my math is off)?
They are not kidding when they call these near-field devices. Life well inside one wavelength is different. You don’t really ever contemplate the idea of a wave at all.
Hm, true, the source is also much closer than a wavelength. And I suppose that even a mechanical oscillator at that frequency (which is quite doable) could respond to an oscillating field.
Thanks everyone for indulging my curiosity and enlightening me.
I love this idea. It sounds brilliantly simple. I don’t know how to tune an antenna to 132kHz but this at least sounds doable. Chances are my neighbor already has his keys back but next time I see him, I’ll mention your solution and see what he thinks of it. He’s a research scientist with an engineering bent so he’ll probably be impressed.