Mysteriously appearing fob...

Have you pulled the rear set out?
It could have gotten caught in the vent under the front seats. Under the rug?
Do you have seat covers? Maybe glove box? Center console?
Maybe in the rear section? By the tire? It's time to start removing stuff!

 

May have to resort to visiting a Toyota dealer. Most information about the smart key system is proprietary, and rightly so. The only commercially available key fob detector relies on another fob that you have previously purchased attached to the existing key fob and detectable through an iOS/Android app. There are only so many places inside a Prius for something like this to hide though. I don't think there is a breach in the time-space continuum.

 

Maybe it fell out of your wallet/purse/pocket/lap?
I've drop mine a few times. There are more cracks and crevices then you think!

 

check the inside rails of the front seats with a flashlight, from front and back

 

The space between the console and seat is a trap that things seem to disappear. A strong flashlight and some serious searching might net you some results. The seat rail is spaced above the floor and objects almost always find their way under that rail.

 

As far as I know, the Smart Key information that dealers have is also available by subscription to techinfo.toyota.com; see my previous posting for more about what Toyota does and doesn’t disclose.

It’s not likely. A dealer could use a Toyota Techstream diagnostic system to confirm that the car is detecting a key, and I suppose you could pay them to disassemble the car to search for it, or to unregister the missing fob, of course.

If I had to look for a key electronically, I’d use a spectrum analyzer, set to show the key’s 314.35 MHz transmissions, and a directional antenna. Few (if any) dealers would have this equipment, and and as @bisco and @BZzap! kindly suggest, a flashlight would be the better tool.

 

But does the key fob transmit continuously, or does it only respond when it is "triggered"? And if so, what does it take to "trigger" it?

 

That's weird: the "missing" fob is being detected only since you have introduced a third one?

 

The key transmits only when a button is pressed or it receives a low-frequency signal (134.2 kHz) from any of several transmitters built in to the car. Indeed, extending these LF signals is the basis of a well-known attack.

 

I would take a guess that key fob emits a very low power RF signal continuously. If your car is locked and you walk up to the door with the key in you pocket, the dome lights are triggered automatically to turn on. Some kind of signal has to be emitted for this to happen. The TPMS transmitters do the same thing.

 

I'll guess that the missing fob is in the car, but found one of the few interior radio dead zones where it isn't detected. Some other Prius owners have found that such spots exist.

But now, it has shifted a bit, into detectability. Or some other interior debris, likely containing metal, has fallen near it and altered the radio wave reflection and interference patterns enough to make it detectable.

 

Of course if you never found the original lost fob, it's possible, maybe even likely it's in the Prius.
But it's incredibly odd, that the fob detected message would only start appearing now. But it could just be a coincidence.

I think the easiest, simplest approach would be the "Red Fern" approach. All you have to do is buy two Redbone Hound Dog puppies. Then raise them, giving them love and attention. Slowly teach them to sniff out and hunt Toyota Key Fobs. I think this can be accomplished by tying one of your remaining fobs to a long string, and having them chase it through forests, fields, and local car dealerships.
Once they have grown, and you are confident of their skill, all you have to do is open your door, and they should very quickly identify the key fobs location, and corner it, allowing you to retrieve it using whatever tools you might need. Saw, Axe, Shotgun....as needed.
All this should be pretty easy, unless one of your neighbors happens to have a Mountain Lion.

 

Funny that the old fob is detected by the car now that you bought a replacement, good suggestion that it has shifted to a detectable position. I bet you'll eventually find it.
I took some hand tools to a friends house to help with his mower, I took a 1/4 in. drive with an 8,9, and 10mm deep well sockets. I put them all on the back seat as I headed home, found it all except the 9mm. This week, I bought a replacement 9mm just to insure that the lost one will show up.

 

It doesn’t; if it did, its CR1632 battery wouldn’t last nearly as long.

Yes, but the first signal comes from the car, which has a much larger auxiliary (12-volt) battery. As Toyota explains in Smart Key System: Course T973B Handbook:

Step 1 – The car is locked. The Certification ECU is pulsing the exterior oscillators, using a low frequency (LF) signal to “wake up” a key if it is brought into the detection area.

Step 2 – The driver brings a smart key within the detection area of a door oscillator. The LF signal from the oscillator commands the key to transmit the Vehicle ID Code wirelessly to the tuner.​

The remaining 28 steps are equally fascinating. The Repair Manual mentions that the interval between the car’s LF pulses is extended, after five days, from 0.25 s to 0.75 s, and after 14 days, they stop entirely, to avoid draining the battery in the car.

Not nearly as often, however. I don’t know Toyota’s interval, but this presentation from Freescale (PDF) suggests that typical intervals are once a minute when the vehicle is moving, and once an hour when it’s parked.

 

ok - equally OT, but jeez - how crappy must 130 kHz propagate, transmitting in milliwatts. Even a matched ¼ wave antenna - back of napkin would need to be what ... >500m? You'd have to be within a couple/few metres just to pick up the signal, much less hijack it. Manufacturers need to do a better job at encryption. Reading another forum, there is at least one auto manufacturer that pays good coin for cracking its security.
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