Apparently he dropped out of sight once his experiments in parapsychology were shown to be a fraud.
That occurred before his work for TSR, though, with Susan Blackmoore's article about his lab being written in 1987.
Seriously? i never heard that one. I wish there was some firm proof one way or another.
There's a thread on the Wizards boards from over a decade ago asking the same question.
There is a report in the thread that Roger Moore claimed that Sargent had been in a bad car accident and was unable to continue working due to his injuries.