My sister went to a necromancer / psychic / medium to find out if our dead and buried father is well and if he is OK. As far as I can understand the Bible, when we die we either go to heaven, to be in the presence of God, or if we’re not saved we go to Hades to await judgment. So, that means that there are no “lost souls” wondering around us on the earth. If that is true, then how can those mediums contact the dead?
You’re correct in saying there are no “lost souls” running around on Earth. The spirits of departed dead are either in Heaven (believers) or Hades (unbelievers).
God does not approve of necromancy (communicating with the dead). He made it a forbidden practice in the Old Testament (Deut. 18:9-13) because it involves conversations with demons.
Those who are called psychics or mediums today are either defrauding their customers (likely in most cases) or are themselves being defrauded by demonic spirits posing as the dead person. Either way, Christians should not be consulting people like this.