Analysis of localised velocity perturbations in the gas disc of HD 163296. Top row: folded velocity residuals computed for ¹²CO, ¹³CO, and C¹⁸O J = 2−1 lines. Middle row: azimuthal and radial extent of clusters identified by our detection algorithm. Regions in bold yellow highlight the accepted clusters of peak residuals whose variances are significantly greater than those in the background clusters. The weighted average of the 2D location of peak residuals within azimuthal and radial clusters determines the centre of the localised velocity perturbations reported in the text. These are marked as green crosses for ¹²CO and ¹³CO. In C¹⁸O, only radial clusters were identified as localised by the technique and, therefore, no perturbation was tagged as detected in this tracer. Bottom row: azimuthal profiles of folded velocities (i.e. those in the top row) to highlight the blue- redshifted, or Doppler flip, morphology of the perturbations detected in ¹²CO and ¹³CO, and of the tentative perturbation in C¹⁸O.