A sensor system and an alerting unit. The sensor system according to the invention may comprise first sensing element configured to measure a first signal indicative of a velocity of a movement of an object and a second sensing element configured to measure a second signal indicative of a direction of the movement, further comprising an alerting unit configured to issue a warning if a predefined relationship between the first signal and the second signal is being violated. The invention further teaches an alerting unit configured to monitor a predefined relationship between a first signal and a second signal and further configured to issue a warning should the predefined relationship become violated.