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.