Using a "voltage sniffer" is a lot like using a torx driver in a square head screw. If everything is exactly right, it works. Otherwise the results are less than desireable.

The only function you should be using a sniffer for is possible presence of voltage. And, if that reading is critical either way it should be followed up with a meter. You shouldn't interpret the distance it reacts from to mean anything definite. In some circumstances you can use them to locate drywall screws.