Lower temperatures can change the time a brew usually takes to ferment, as the yeast is working slower than normal. It is quite common for intermittent bubbling activity to continue through the airlock after the completion of fermentation. This is caused by CO2 gas remaining in the fermenter and does not necessarily indicate that the brew is still fermenting. If the brew becomes infected an extended fermentation can occur, resulting in an off smell and sour taste. Also, white film usually develops on the surface of the brew.
The only accurate method of establishing when the brew has finished fermenting and is ready to bottle is by taking readings with the hydrometer and when you have two readings the same, 24 hours apart, it is safe to bottle.