I made a braggot on 4/22/12
9lbs orange blossom honey
4oz cascade @ 60
4oz cascade @ 0
By 5/25/12 it was down to ~1.020 and wasn't showing any activity so I racked to carboy for bulk aging.
Fast forward to July - I was gone for ~3 weeks so my house temp was relatively stable. When I got back, I opened up windows for a few days to cool down the house a bit, and a couple days later the braggot appeared to be fermenting again - enough that I had to rig up a blow off tube. I thought it was just offgassing because of temperature swings (and probably a bunch of CO2 stuck in all that hop gunk), but this went on for a couple weeks.
I checked gravity and it was still 1.020, and it smelled and tasted fine (well, as fine as a young braggot should
Obviously, if the gravity isn't changing, fermentation isn't happening, but this seems to be a long time for offgassing to occur.