Here they put some in the zombie kill routine, but I don't think there is anything special about zombie kills. It could have been anything: picked up ammo, loading a map, pass a certain event, on a timer, during a save, all of the above, etc...
I don't have a deep knowledge about the technology involved but from what I've seen modern DRM like Denuvo are based on virtual machines and are very flexible, allowing all sorts of checks at many points. Done well, there is little overhead, but some overzealous publishers do frequent lengthy checks, killing performance.