Last Thursday I bottled a kit brew that had been in the fermenter (in a fridge at around 18 deg. C) for 32 days, mainly because there was still airlock activity. I don't check the SG to avoid stirring up the sediment by shuffling the fermenter around in the fridge, a system that's worked so far (30-odd brews).
I was starting to worry so checked with my highly knowledgeable local supplier. He said the brew should be "well and truly done", advised bottling immediately, and said there should not be much to be concerned about. He couldn't offer any real reasons as to why activity was continuing but suggested it was maybe some 'enzymes'. (Shouldn't no sugar = no activity?)
I did a few taste tests through the bottling process and there did not seem to be any 'off' flavours. In fact, they tasted quite nice. Hopefully the end product will taste as good.
My main concern now is 'bottle bombs'.