Ah... OK, but the key point is that despite the fact that there appears to be no bubble activity in the airlock the sugar is being eaten up by the yeast. And so this is yet one more datum point that shows that airlock activity is meaningless. When it does not occur it might be because there is a poor seal in the cap or the airlock and when it does occur that does not necessarily mean that yeast are doing anything: it could be that particles are precipitating and that precipitation is causing the CO2 to nucleate and so leave the liquid or it could point to changes in ambient temperature or air-pressure both of which can affect the amount of CO2 that can be dissolved in a liquid.
Oh - and for the record, I tend to use a bucket as my primary and I cover the bucket with a towel to keep out dirt. I use a bung and an airlock after I have racked and I rack at the end of active fermentation so very little CO2 is being produced by the yeast. If I want entertainment I don't watch my airlocks.