It seems to appear after I've worked on a file for a little while in a session. I can't find it on Google, probably since I'm using the term "bug", but "insect" doesn't work either. :(
It shows up to the left of the filename in the tabs below the editor, always only on that one file I'm focusing on (by chance, I only do a lot of work on one file in each session).
There's no mouse-hover caption, or apparent documentation on it! The insect seems to appear after 10-20 minutes or so.
Edit: I promise I'll upload a screenshot as soon as it comes back. :) Edit: Uploaded- the yellow hovering box is when I hover over the bug, which indicates there's no special description.