I suspect that there is still refrigerant in the system, however even after recovery a little bit of popping and hissing is not unusual as residual refrigerant boils off. If a hiss does not stop after a second of loosening a fitting, evacuate and just the refrigerant leak out. Refrigerant can give you frostbite rather easily if it bursts out. The liquid is also very likely oil of which there should be a fair amount in the system, be ready to clean up a bit of a mess.
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