HA! I should have shut my damn mouth.
Went to open it today, and the top came off in my hand. It looks like it and the two hinge-things are held in place by a single bead of black RTV. So I guess I have to find some, scrape off the old, and glue it back.
I ordered the "PC-Auto Bond" as indicated, above. More expensive than most, and took a while to get from Amazon (since nobody locally sells it).
Of course the left cover came off, since I use that pannier 10 times as much as the right pannier.... I decided to try pulling on the right cover. Sure enough, that popped right off. Figured, since the adhesive on both are the same age. Plus, this bond stuff is expensive and has a mixing tube that can only be used once, so I want to make sure I didn't have the right one fail a year from now or something.
I used a razor blade to remove as much of the old black sticky adhesive as possible on both the latch and cover and the little hinge pieces. Then I used the razor to scratch hash marks all over them for adhesion. Squirted the new filler-epoxy using the same pattern as the factory (it was not as thick as I had hoped/expected) and placed the covers back on. Now only time will tell.
Photos attached