I have an occupancy detector switch controlling a split 15a rec. (top half always hot, bottom half switched. ) A very low resistive lighting load is plugged into the rec. When the switch is set to Auto, the light is always on (low power). I swapped out that load with an incandescent lamp and the switch operates as expected. My meter is reading 2-3 volts from the recep when the switch is on Auto and no motion is present. Circuit is grounded all the way to panel.
I figure my options are: 1. Put the switched recep in its own box. It's in a 2gang box with another recep. 2. Find a way to add resistance to the low light load. 3. Try another switch.
The sensor is deriving power from the AC line through the load. The sensor needs power all the time to be able to detect motion, and as a result lets a few mA leak through the load to power itself. With incandesant lighting this is unnoticable and not an issue. If you read the spec. sheet these sensors typicaly call for connection to permenant incandessant fixtures (not plug and cord connected). With an incandesant fixture (there may be a minimum wattage specified) it will work fine.
I know this is not a solution but it should atleast help you understand the problem. It is nothing to do with your wiring methods.