A system for detecting defects in a print system may include a print engine to print an electronic document (a reference image) and yield a printed document. The system may also include an image sensor to scan the printed document into a scanned electronic document (a target image). The system may include a processing device to detect defects in the printed document by analyzing the activity level in the reference and target image. The system may identify a quiet area/pixel in the reference image based on the activity level, check the activity level of the corresponding pixel in the target image, and classify the pixel in the target image as defective if the activity level of the pixel in the target image exceeds a noise threshold. The system may additionally swap the reference and target image, repeat the detection steps and combine the detection results with those before the swap.