The retrieval of voluminous defects in a non-magnetic metal plate from maps of time-harmonic anomalous magnetic fields in air due to a nearby low-frequency source is considered. The wavefield is modelled from a rigorous full-wave vector domain integral formulation and the inversion is performed via a binary version of the so-called contrast-source, gradient-type technique. The functional analysis is outlined and the efficiency of the approach is discussed in the light of inversion results obtained with synthetic data in three typical defect configurations.