Here is the history page for the facility:http://dmh.mo.gov/slprc/history.htm
It opened as the St. Louis County Lunatic Asylum on April 23, 1869. It was the 2nd in the state of Missouri to serve the mentally ill population.
This sentence appears about halfway down: "The next overflow crisis occurred in the early 1920s which precipitated a separate building being erected for attendants' quarters."
So it looks like at least some of the hospital attendants lived on the campus. That may be why your relative was there.