I just had this happen to me on my five year old FEC-100 today. The other day I saw smoke coming out of the hopper, but it quit as the temperature got up to the 250 mark so I didn't think much about it. Today I did a 4.5 hour rib cook, forgot to turn it off so it went into hold mode for a little over three hours. When I remembered to turn it off I opened the hopper to see what was left of my pellets and there was quite a bit of water condensation on both the inside top lid, and sides of my hopper. Also more ash than normal inside the cooker.
We've had a ton of rain recently, but my FEC-100 lives under a covered patio with the cover on most of the time. No condensation in my hopper before the cook.
Just saw this as I was spooking around and decided to reply because I had the same issue.
Remove the chimney from the back of the smoker and clean it out. You are probably going to find that the chimney is choked with ash and condensed animal fats. You need to clean this out periodically, frequency depends on how much you use it.