A sensor surveillance system (100) and a method for determining possible geographic positions of at least one assumed undetected target (1a-n) within a geographic volume of interest (200) is provided, wherein for a first point in time ti the following steps are performed: dividing the geographic volume of interest (200) into sections (10); assuming the existence of an assumed undetected target (1a-n) at a geographic position within each section (10); and initiating the creation of a pattern (2) defining at least one possible geographic position of the assumed undetected target, said pattern extends at least partially around the geographic position of the assumed undetected target (1a-n); wherein the geographic extension of said pattern is determined based on: the category of the assumed undetected target (1a-n); and the amount of time that has passed from the first point in time t1. Further, for a second point in time t2 the following steps are performed: determining geographic locations (205) within said geographic volume of interest (200) from where sensor signals show absence of targets; and removing the pattern (2) from the geographic locations (205) from where sensor signals show absence of targets.