Why am I not seeing Cook's distance lines in my Residuals vs Leverage plot?

My residuals vs. leverage plot:

shows a legend for Cook's distance lines but doesn't actually show any, though it does highlight three potentially influential points.

I tried very simply plot(leave_MRM1,5), so I was expecting to see them. This is in a Jupyter R Notebook.

• the most likely answer is that all of the Cook's distances are well below 0.5, so the lowest contour line doesn't appear within the plot boundaries; what is max(cooks.distance(leave_MRM1)) ?
• it is a very common misconception that the labeled points in base-R lm diagnostic plots have somehow been flagged as potentially influential points; in fact they are simply the three most extreme points (presumably those with the largest Cook's distance, even if this is not particularly large) in each plot: from ?plot.lm:

id.n [default = 3]: number of points to be labelled in each plot, starting with the most extreme.