The usual method of inspecting elements didn't work here. Right clicking had apparently been disabled on elements within the drop down menu. Using the context button on the keyboard as an alternative to right clicking didn't work – the element wasn't selected.
Here's how I eventually managed to track the dropdown menu down.
Find a DOM element which you think becomes a parent element of the dynamically created element you want to inspect
Right click on the element, and select Break on > Subtree modifications
Back on the page itself, trigger the insertion of the dynamically created DOM element by hovering, clicking, or whatever you need to do
Hover over the variables in the script to get more information about them
When you see a likely looking one, right click, and select Reveal in elements panel
This will reveal the element.