The lifetime of positrons in a lattice is strongly affected by structural defects even if the concentration of the latter is well beyond the limit of detection by classical methods. The lack of a repulsive center in a vacancy acts as a trap for the positron which then annihilates itself with reduced probability due to the lower electron density at this site. The existence of a component longer than the bulk annihilation thus reflects the presence of vacancies. The behavior of this long-lived component in ZrN films sputtered onto stainless steel substrates was monitored over a period of 18 months. During this time, its value decreased steadily by a factor of 2-3 depending on the negative bias used during the sputtering. No significant changes in the color were observed during the measurements. A consistent explanation is the breakup of polyvacancies or voids leading to a higher concentration of monovacancies.