How is an electronic leak detector used to locate a refrigerant leak in a vehicle's engine bay?

1. By spraying soapy water on the AC hoses and watching for bubbles
2. By moving the detector probe slowly along the underside of AC hoses, connections, and components, as refrigerant gas is heavier than air
3. By connecting the probe to the battery ground and checking for spark arcs
4. By inserting the probe directly into the engine radiator oil filler neck