The invention relates to a sensor device (100) and a method for the detection of magnetic particles (1) in a sample chamber (2) with a contact surface (11). The sensor device (100) comprises a sensor unit (120, 130) for detecting magnetic particles (1) in a target region (TR) and/or in at least one reference region on the contact surface. Moreover, it comprises a magnetic field generator (140) for generating a magnetic field that shall guide magnetic particles to the contact surface. With the help of these components, an “auxiliary parameter” is determined that is related to the magnetic particles (1) and/or their movement but that is independent of binding processes taking place in the target region between magnetic particles and the contact surface. The auxiliary parameter may for example be related to the degree of mismatch between the positions reached by the magnetic particles (1) under the influence of a magnetic field and the target region (TR). The evaluation results can be used to validate and/or correct the measurements obtained in the target region (TR).