one of the reasons why air lock activity isn't a good indicator of fermentation is that the CO2 may be escaping elsewhere. if your lid doesn't seal properly or the air lock isn't jammed in properly, it will be easier for the gas to escape that way instead of pushing through the liquid in the air lock. so no bubbles doesn't mean that there isn't fermentation happening. just means there are no bubbles
and on the flip side, the presence of bubbles doesn't mean there is active fermentation. it just means CO2 is being release, and there are reasons other than active fermentation to explain the release of CO2 from the brew (atmospheric changes, temp changes, being shaken/bumped, etc).