I have the same thing happening with by boat. I know on Polyester fiberglass construction there is post cure that continues on for years, and is hastened by heat. I am reasonably confident that happens with epoxy also. I probably used one extra fill coat of epoxy, and five coats of Epifanes. I leave the boat in a small garage which does get hot in the summer. I think it was out over night two times, but other than use, it is inside. I am the only one that sees it, so I am not concerned.