SHADOW CREATURES AND THE MODERN WORLD
Humans cling steadfastly to the reality that they wish to be true, resisting every piece of evidence that suggests that the world isn't the way they believe it to be. Only a relative few make up the "aware minority," those humans who have experienced a Shadow event and seen it for more or less what it was. The majority, the mundanes, find comfort in translating kobolds with swords into knife-wielding muggers and malicious spirits into tricks of the light. A magical effect might be fireworks or a weather peculiarity. Never underestimate the human mind's power to deceive itself and maintain a level of comfort.
Those who subconsciously delude themselves do so for the sake of their fragile sanity. Those who know and understand at least some aspect of the Shadow tide maintain the secret to avoid ridicule or investigation, as well as to protect the public illusion of a sane, mundane world.
In addition, since Shadow creatures really don't belong to this reality, they have a tendency to slowly fade away. Some of them disappear within 1d4 rounds of dying (being reduced to -10 hit points
), and none remain in this reality for more than 1d4 hours before the tide of Shadow reclaims them.