Thanks for the responses. Collitchboy, there was no rapid difference in temperature, it was just left alone for a couple of months without really checking it. (The one good thing about Florida, though it's usually too hot!). DoctorWho, the solution was mixed to the correct ratio, so you and the guy at the LHBS are in agreement in your assessment. The guy at my LHBS also suggested that the fluid in the airlock may have evaporated. Has anyone heard of this phenomenon occurring? In any case, I have two new batches going and am keeping an eye on them. I also purchased two "S"-type airlocks to blunt this issue in the future by using them in secondary and for bulk aging, though I am still using the 3-piece airlocks for primary.
LHBS guy also said that commercial cideries use airlocks when aging. Is that true? I assumed that like wine, cider could be stored in sealed barrels.