Brewed my second batch of chocolate stout last month. Starting gravity was 1.056. Left it in primary for about 17 days, krausen all re-absorbed, but couldn't check gravity (hydrometer broke, didn't replace it in time). Racked to secondary, where it has been for about 2 weeks (I am going to give it another 3 before I will bottle it). Just went down to the basement, and I noticed that there is slow but regular bubbling in the airlock, and foamy bubbles on the top of the brew. Secondary is a Better Bottle, and I am using an S type airlock (I use a 3 piece on the primary). It's been sitting at a pretty constant 68 degrees. Should I be worried that it somehow got contaminated?