Caging the transmitter [or receiver] in an extremely thick case of thick lead is tantamount to an atmospheric condition, albeit ex-extremis. There would effectively be zero measurable signal. In a superhetrodyne receiver (I.e. commercial FM radio), the point at which there is no signal is the point at which the IF Filter (Intermediate Frequency) can't squelch any signal from noise. Your oscilloscope would then show no meaningful signal at the IF Amp and the Demodulator stages. A listener to a radio receiver / transciever hears the signal fading in and out when it is marginal to IF Filter sensitivity. This can be due to atmospherics or noise (i.e. perhaps a train engine on an electrified rail somewhere inbetween nodes). However, if the signal is significantly below the IF Amp sensitive threshold (even after pre-amping) then you won't hear diddly squat... well, apart from static.