Remember that fermentation is exothermic, meaning it produces heat during the process. That means that the internal temp of the fermentation is typically 3-8F higher than the ambient temp.
SO, you likely had an actual fermentation temp of 69-76F. At those temps, it's very possible that your beer fermented overnight and you just missed it!
Like others said, it's also very possible that your bucket top is not airtight. No big deal, but the CO2 will escape through the path of least resistance. If there is even a small space where the bucket lid is not airtight, the CO2 will escape there instead of going through the airlock. Not really even a sanitation concern, so no need to worry, but it's quite common, so that could be another thing that's going on.
In the end, at this point just practice good hydrometer reading and finish the beer out. It should be just fine.