Does anybody know how or why items would disappear from the Track Menu List? There are a couple props in my scene that I need to find on the timeline so I can remove unwanted keys that are causing them to move, but how do I open their places on the timeline if I can't click on their corresponding entries on the Track Menu List? 

Could be a bug, but I would not know because I never work with "Track Menu List". I use "Object Related Track" button to bring items to the timeline.
Too lazy to type it over, but you could glance at iClone Help, or at this post where I laid out my workflow: https://forum.reallusion.com/375400/RE-Camera-placement-on-timeline#bm375692

OK, thanks.