As has been said in this and other threads, and on other forums- the airlock is not an instrument that is particularly accurate in measuring fermentation. A lot of the standard "Ale Pail" bucket fermenters have no lid seal, it just snaps onto the bucket rim.
If you want to eliminate this type of leak, US Plastics makes buckets (their 7 gal. bucket is virtually ideal for 5 gal. batches) with a real neoprene 'O'- ring in the lid, which seals the bucket. I got three of these, with lids, for about $33 delivered. Drilled a hole in the lids with a 1/2" paddle bit and installed grommets available at fine homebrew stores everywhere.
The only foolproof way to see where your fermentation has gone is to pop that lid, take a sample and put the hydrometer in.
“Malt does more than Milton can / To justify God’s ways to man”
-A. E. Housman (1859–1936). A Shropshire Lad , 1896.