Captivate 2017: shape appearing on slide but not on timeline

Hello. 

I have an issue with a shape appearing on a slide but it's not on the timeline. How do I delete it when I can't find it on the timeline?

I thought that I may have somehow added it to the master by mistake but when I checked the master view it's not appearing there either.  Also when I insert a new blank slide (my entire project is made up of blank slides) the shape automatically appears on that slide.

The weird thing is that I have this shape on all of my slides (its a rectangle that I'm using as a border around a videos I've inserted) and on the first few slides it appears just fine. But starting several slides in its suddenly appearing even if it's not on the timeline.

I've rebooted Captivate. I've rebooted my computer and it's still appearing on the slides.  Any idea what's going on?

Thanks, Marion