It could surprise you to learn that yes, hair can grow back long after laser treatments.  Although laser beams destroy a percentage of hair follicles during treatment, they typically only damage them.  The result is temporarily disablement from growing new hairs.  They can eventually heal and start to regrow hairs months or even years after treatment.
Some people may need touch-up treatments every so often to maintain their results.  In addition, some people may experience new hair growth due to hormonal changes, medication use, or other factors.  It is important to note that laser hair removal is not the same as electrolysis.  This is the only method of hair removal that has the approval of the U.S. Food and Drug Administration (FDA) as being permanent.