The truth about Mark's wife in Severance was a shocking moment, but there were hints to the reveal throughout the first season. Though Mark's wife was initially thought to be dead, season 1, episode 7, “Defiant Jazz” revealed she was Ms. Casey, the Wellness Counselor for the employees that work on the Severed Floor. Before the reveal, many viewers appear to have seen the moment coming, having posted extensively about it online alongside the many other Severance fan theories. However, Severance has dropped many clues to the twist about Mark's wife throughout the earlier half of the show.
https://screenrant.com/severance-mark-wife-ms-casey-gemma-identity-clues/