The foil does not even have to be on the tube; a high impedance conduction path to the tube seems sufficient.
EDIT: it looks like trying to measure a voltage across one hands as I did here through the tube (photos removed) may indeed produce positive results and should be expected. In any case, I still got a few mV as shown earlier on without touching the tube with my hands and with a thin separator to the Al foil. That should be more similar to what Alan Smith reported doing, albeit to a lower extent.