Air conditioning coil has been pressurized and dipped in water. As you can see this coil has many microscopic leaking sources on copper tubing.
Finding a leak source in air conditioning system can be hard or easy all depend on size of leak . A large leak can be located easily, like cracked pipe or broken but a small one can be hard and very time consuming, since all spots and piping are not readily accessible. A technician have to suspect all welds and connection. you can see on this video a safety valve is leaking since the back group is shiny there was not trace of oil residue.