The scatter plot in Figure 3.2 for

The scatter plot in Figure 3.2 for lag-3 (lower left plot), for
example, has a distinct lower-left to upper-right slant supporting positive lag-3 autocorrelation,
but an outlier (highlighted) probably keeps the lag-3 autocorrelation from reaching statistical
significance. Influence of outliers would not be detectable from the acf alone.