These tests shows how iron in the the area effects the detection of your target, i have not found a detector yet that will find a non ferrous piece of metal when iron is on the surface above it, unless the iron is touching it.
Test one- Nails placed directly on top of a Hammered Medieval penny,( most detectors fitted with a wide scan coil get this)
Test two- Hammered Medieval penny placed next to a iron bolt
Test Three -Hammered Medieval penny placed among 20 rusty nails
Test four- 20 pence