An apparatus and method for detecting a relatively narrow predetermined pattern, such as a trigger mark, on a moving printed product uses a plurality of sensor elements arranged linearly in an array and a switching apparatus for selecting a properly located subset of the sensor elements for detecting the predetermined pattern. During the operation of the apparatus, only those signals from sensors in the selected subset are checked continuously for the occurrence of a signal pattern corresponding to the predetermined pattern. Each time the predetermined signal pattern is found in the output signal from a sensor within the selected subset, a detection signal is generated. The lateral shifting of the predetermined pattern over time may be monitored by the selected sensors, and the selection of the subset of sensors for the continuous evaluation may be changed in response to the lateral shifting.