[thick18cm]

just to clarify the wording of your essentially correct supposition, the radio signal emanated from across the street "completely external to the device", but when this signal encountered the device, the device resonated (like an opera singer breaking a glass across a room, or even like your eardrum vibrating while listening, except EMF waves not sound) and the listeners across the street could detect the signal from this modulated resonance... so ultimately, not "completely external" to the device.

this type or degree of "vibrating resonance" is passive "signal electronics" rather than the active power electronics inside RFID, which uses a stronger signal resonance to generate power to power up an active broadcast and computing circuit.

it's not entirely unlike bouncing a laser off the outside of a window to detect the conversation inside, though that higher tech and frequencies means the system does not need to be tuned to a resonant frequency.