Here and there on my model there’s a sudden increase in vertices appearing out of nowhere.
No subdivision modifiers have been appied. This is the second time this happens to me after Steam decided to update my Blender to 2.81a. I have also not done any CTRL+R’s accidentally.
Subdivision modifiers can’t do selective subdivision (not like that in any case), and certainly this can’t happen on its own. It looks like you had some face or edge selection and hit Subdivide in edit mode (by accident?). IIRC there’s no hotkey associated with it, but it is in the right-click context menu, so still could be called with an unintentional key press when that menu is open.