SUS304 is non-magnetic materials in condition of solution state from austenitic stainless steel,but
steel wire for springhave weak magnetism because parts of steel wire becoming martensite by cool processing.
SUS316is non-magnetism because magnetism increases by cool processing is small compare with SUS304.
Magnetic strength of magnetic attraction is too different between stainless wire and piano wire, also usually the surface color of stainless wire is more gloss than piano wire.
Large-size spring which is hot coiling,do quenching heat treatment after usual heat treatment because of necessary hardness. Also usually surface color of stainless wire is more gloss than piano wire.