What to do when measuring voltage on nails in the wall?
Your photo shows the Red lead plugged into the 10A socket of the meter.  Therefore you are measuring the voltage between the COM socket and the HIGH socket on the right which is probably just due to stray 60 Hz fields. Try plugging the RED lead into the HIGH socket and the BLACK lead into the COM socket and repeat your measurements.  In all likelihood, they will still be small and just indicate the present of 60 Hz fields from the power lines.