An air lock is used to relieve pressure in the bucket so it doesn't become a bomb. That is all. It does not tell you when your brew is done, your hydrometer does. The yeast you used might just work quickly, RDWHAHB. And pulling the lid off should be fine, as long as you weren't doing it in a functioning wood shop. There is a layer of Co2 over the beer, it will protect it from the air.