Hopefully, it is just CO2 offgassing in a warmer environment. Next time I would ditch the secondary (use of a secondary is pretty outmoded advice for most beers and just opens up more chance for oxidation or contamination). The key on final gravity is that the gravity is stable and not changing over a few days - that is what tells you the beer is done. Target gravity is helpful, but essentially meaningless.
Best of luck with the beer and I hope it has not developed a contamination - continued airlock activity for such a long time does not bode well, unfortunately. Is there any unusual looking pellicle forming on the surface?