Was just having a look into the Biltmore estate and came across an old blog which mentions secret tunnels etc
I also recommend watching “The Private Eyes” starring Tim Conway and Don Knotts before and after you visit. It shows a lot of the house (before certain changes were made and a lot of the sercret passages).
https://pastaqueen.com/blog/2010/09/turn-of-the-century-athletic-equipment-um-last-century/