InDesign adds a blue vertical line in the margin when I edit anything (text, paragraphs, etc.)

On some documents, when I edit something, InDesign adds a blue vertical line to the left of the text frame. What is this, and how do I turn it off?  It does not seem to be associated with the Preferences > Composition settings. I have tried comparing a document that does not do this with one that does, but so far, I haven't seen any differences between the two. Here's what I'm talking about - the Screen Mode is Normal:

It's not a show-stopper; just really annoying. 🙂