1. A method for detecting a three-dimensional item inside an inspection object, the method comprising: detecting edges of a plurality of items contained in a transmission image of an inspection object in order to produce an edge image, at least some of the items being three-dimensional; and detecting within the plurality of items a three-dimensional item having at least one symmetry property by determining a symmetry line of the items contained in the transmission image, the three-dimensional item having at least one symmetry property comprising a bottle, the symmetry line determined by detecting pairs of edge picture elements of the edge image that are positioned symmetrically to each other relative to the symmetry line that is to be determined, wherein, in the determining a symmetry line in the edge image, the only edge picture elements that are taken into account are those for which the symmetry line in the transmission image lies in the image region of the three-dimensional item to which the edge picture elements belong, determining, in the respective transmission images, a direction for the respective symmetry line by determining a bottom of the bottle and a neck of the bottle in the respective transmission images based on a difference in thickness between the bottom of the bottle and the neck of the bottle; and producing an output classification associated with the item detected.