What parts of a home air conditioner unit could leak freon?

Just about everything: The compressor, the condensor, the evaporator, the expansion valve or capillary tubes, all of the piping, the reversing valve if it is a heat pump, the valves, the service ports, the filter/drier if there is one. Refrigeration service techs have a special 'sniffer' used to find the source of a leak.