Welcome to the forum.
Wow, that looks like a square Shape w/ a thin Stroke and no Fill.
I can see Video Tracks 1 - 3, and nothing is appearing above the Clip on Video Track 1, but just to be sure, are the other Video Tracks (if there are more) also completely empty?
[Edit] nevermind that question, a close look at the scroll bar for the Video, appears to be at the top of the vertical stack - looked like there might be more Video Tracks, but a close looks appears to indicate that no Tracks are above. That rules out a Title with a Shape. You got me on this one.
Message was edited by: Bill Hunt
Thanks for the effort! I'm probably just going to have to back track and swap out the clips.
If you can nail this one down, please post.
I have thought, and thought, but cannot come up with any idea how you would get, what you showed. I have never had it, nor have I read about it, but that does not mean that no one else has encountered it.
Good luck, and wish that I had more to suggest,
Looks very much like a preview box that can be left in the rendered image when using the DeNoiser Effect in After Effects... but you say that it is not in the original footage so that eliminates that.
Can you open the Effects window and expand the effects for a screen grab?