It is provided a method for determining when a break-in attempt is in process. The method is performed in a break-in determiner and comprises the steps of: determining that a first vibration condition is tme when a vibration parameter associated with a barrier is greater than a first threshold, wherein the first vibration parameter is obtained from measurements from an accelerometer; determining whether an acceptable activity condition is tme or not, such that the acceptable activity condition is true only when there is an auxiliary signal indicating acceptable activity comprising determining that the acceptable activity condition is tme only when a time difference between determining that the first vibration condition is true and receiving the auxiliary signal is less than a threshold duration; and determining that a break-in attempt is in process when the first vibration condition is tme and the acceptable activity condition is false.