Difference of the relative astrometry of the bright point-source lying on the spiral-like feature (see Fig. 4) between the HST/STIS epoch (2013-06-20, used as reference) and the SPHERE/IRDIS epochs (2016 to 2019, see Table A.1). The blue solid curve shows the expected track of a background object, based on the parallactic and proper motions of the primary star HD 120326. The point source is clearly identified as a background object.