I think these tools work by measuring the rate of conductivity for each metal.

https://streamsystems.com/stream_web...fo/hotshot.htm

https://bossshotshells.com/shell-detector/

Fines and penalties will vary between Federal and State regulations. If caught on federal land your gun, boat and truck can potentially be seized. State fines have a large range.

I wonder how sensitive it is to pick up lead on old lead holding hull used for bismuth or steel reloads?

Trace amounts of lead will not be detected. These devices are just metal detectors. They apply a magnetic field and measure the response, which is a function of mass and electrical conductivity. In your case, the mass is near zero (trace) and the conductivity of lead is low. The signal from the brass on the end of the shell would overwhelm any noise from trace amounts of lead in the shell.

-Victor