I have currently run a 15 amp circuit with 12awg wire. I have used plastic boxes to house the wires for a switch and junction. When I test the current, I get a signal along my neutral wire. As I test other circuits, I do not. I have tested the neutral wire at the service panel, and do not get a signal. Is this because I am using plastic instead of a grounded metal box?

If this is a contactless "voltage detector" kind of device, then the answer can be yes. In the service panel it is really at ground potential. It really has to do with how far away you are to a non-grounded wire and the effectiveness of any metal shielding.

If you put a grounded metal shieled between the detector and the current carrying wire that's large enough then you won't detect anything.