As to how the little "foot" got out, I can tell you how mine escaped and stayed nearby so I found it. I've got a cat that likes to walk across the keyboad and Magic trackpad when I'm not around. I've created a low "stand" for the monitor with an upper shelf for the MacBook and a lower shelf (on the desk) to slide the keyboard and MTP under the MacBook. I'm sure that sliding the MTP back and forth over time caused the silicon "foot" to slide out. I'm now turning the KB and MTP over on their top and sliding them into my out-of-the-cat's-way hidey hole.
THE REINSERTION: The suggestions in this discussion about prying up the bottom of the trackpad worked fine for me but the foot popped out again a few days later. Couple of suggestions.
When prying up the bottom plate, pry from the side of the trackpad not the edge opposite the battery end. I've pried it up a few times now and one time pried from the side oppostie the battery pack and the tab that snaps into the top broke off. The bottom still goes back on tightly but that tab is now history. Message: Pry from the side and work the tab on the bottom out.
REPEATED POPPING OUT. Just today, I've put a very small dot of Elmer's glue at the joint of the rubber foot and the bottom on what I would call the top and the bottom of the foot - when holding the trackpad vertically with the batteries up. The track pad action seems to be fine but if I suspect the small dot of glue is preventing the mouse-click action from working, I can carefully scrape off the glue. However, I'm trying to keep the rubber foot from popping out again for the 10th or so time.
SURFACE OF DESK ISSUE; After the glue dried, I tried sliding the trackpad around in a "normal" fashion and could feel the rubber feet "skittering" a bit on the surface of my desk. The desk is a composit wood with plastic surface as many modern desks. Wanting to reduce the friction of the desk surface, I walked around my basement looking for brainstorms and noticed the old can of car wax - remember when we used to wax our cars? Turtle Wax it is. I applied a light coat of Turtle Car Wax to my desk surface as I would to a car fender, then polished it with a soft cloth. The desk surface does not look nearly as good as my old '47 dark green Pontiac, BUT the rubber feet on both the track pad and the bluetooth keyboard now slide almost effortlessly. I don't feel those rubber feet skittering now. I'm hopeful that the glue and the light car waxed surface combine to eliminate, or at least reduce, the rubber feet pealing themselves out of their hole onthe bottom of the trackpad.