It is a result of the mechanism used for operation. See https://en.m.wikipedia.org/wiki/Superheterodyne_receiver especially the section on Local oscillator radiation

Most radio receivers transmit a very weak signal slightly higher than the carrier frequency of the station tuned as a result of how they enhance received signals. Look up superheterodyne for details.

It's a side effect of how tuning works (local oscillator), and the signal is extremely weak, very little of it radiates out due to some components / PCB traces behaving as an accidental (poor) antenna.

Still, with good enough equipment it can be detected even from some distance.

Check this out: https://scholarsmine.mst.edu/cgi/viewcontent.cgi?referer=&ht...