Knees do some strange things as they are healing. I must admit that I don't recall another member who reported something like this. It's VERY helpful that you posted a photo. It would be good if you could post additional photos when this occurs and possibly it will help in figuring out just why you would have this raised line on your leg.
I don't think it's anything to be concerned about, though. You aren't having any pain or redness associated with it. It doesn't last long and occurs in different places around your knee. I'm assuming the rest of your recovery is progressing normally as well. Perhaps someone will see your post and chime in with something similar. But in the meantime, please try not to worry about this happening.