Task Force 3 provides the plausible sighting

The counterfactual pivots on Task Force 3, historically conducting gunnery training with USS Indianapolis and four converted destroyers. Changing the exercise to counter-raider and anti-submarine work gives a doctrinal reason to add USS Phoenix, pairing an 8-inch cruiser with a 6-inch cruiser and raising the force's scout-plane strength from three to seven. Cloud and squalls, Japanese radio silence, and the absence of Japanese radar give an American scout a plausible chance to see the carriers without being seen. The aircraft could radio the sighting, continue observing or photographing the formation, and then return while the cruisers shadowed from a distance. Once that report had been relayed, attacking Task Force 3 could not restore surprise and would instead begin the war on 4 December.