Unity Technologies have released an excellent blog post covering this topic, which can be found at https://blogs.unity3d.com/2016/12/20/unitytips-particlesystem-performance-culling/.
The basic idea is that all Particle Systems are either predictable or not (deterministic versus nondeterministic), depending on various settings. When a Particle System is predictable and not visible to the main view, then the entire Particle System can be automatically culled away to save performance. As soon as a predictable Particle System comes back into view, Unity can figure out exactly how the Particle System is meant to look at that moment as if it had been generating particles the entire time it wasn't visible. So ...
