There are numerous precedents for imaginary events. Consider the Second Gulf of Tonkin Incident, a response to an imaginary attack:
"Over the next three hours, the two ships repeatedly maneuvered at high speeds to evade perceived enemy boat attacks. The destroyers reported automatic-weapons fire; more than 20 torpedo attacks; sightings of torpedo wakes, enemy cockpit lights, and searchlight illumination; and numerous radar and surface contacts. By the time the destroyers broke off their "counterattack," they had fired 249 5-inch shells, 123 3-inch shells, and four or five depth charges."
It wouldn't take much for people to imagine a drone incursion - perhaps as little as a bird or escaped birthday balloon.