Local piezoresponse hysteresis loops were systematically studied on the surface of ferroelectric thin films of BiFeO 3 grown on SrRuO 3 and La 0.7 Sr 0.3 MnO 3 electrodes and compared between ultrahigh vacuum and ambient environment. The loops on all the samples exhibited characteristic asymmetry manifested in the dif-ference of the piezoresponse slope following local domain nucleation. Spatially resolved mapping has revealed that the asymmetry is strongly correlated with the random-field disorder inherent in the films and is not affected by the random-bond disorder component. The asymmetry thus originates from electrostatic disorder within the film, which allows using it as a unique signature of single defects or defect clusters. The electrostatic effects due to the mea-surement environment also contribute to the total asym-metry of the piezoresponse loop, albeit with a much smaller magnitude compared to local defects.