A baseline-free defect localization method in thin plates is proposed and tested. In this proof-of-concept work, a steel ball pressed against an aluminum plate is used to mimic a surface contact defect. The technique takes benefit of a repetitive nonlinear pump-probe interaction with a backpropagation imaging algorithm. High-frequency probe waves are periodically emitted by a piezoelectric patch transducer glued to the plate. Propagated flexural waves are recorded using a distributed array of transducers. At the same time, a continuous low-frequency pump vibration provided by a shaker fixed to the plate modulates the contact state. By combining multiple probe signals, the contact can be successfully localized. Contrast of the localization images is finally improved by a factor of 3 to 5 by implementing a modified version based on synchronous detection of the imaging algorithm.