That's a good point.
As an example, I decided to rack some wine today. One wine has been in a carboy since December, 2013, so it was about time..................
Anyway, it was moved out of the corner of a cool room, and put on the kitchen island for me to rack it. I let it sit for about an hour, and the airlock started bubbling pretty regularly.
Fermentation has been finished for over a year- but it was a bit warmer in the kitchen than in a corner of a cool dark room.
Sometimes a bubbling airlock is an obvious sign of fermentation- but often it is not. Sometimes the barometric pressure changes and the airlock bubbles, or the temperature rises, and it bubbles. It is not important at all.