I was in the middle of a Savannah Biome and next to my base when I saw a Vindicator naturally spawn in out of nowhere. I have never encountered a Pillager Leader, and have never started a raid, nor have i found a Woodland Mansion. The three images show the nearest village to me, and no raid bar being visible, the Vindicator itself, and the fact that this was an entirely survival world. The Vindicator did function as normal, and attacked me with their axe once they saw me.