Theory: Season 3 Episode THE BORDER caused the loss of the mansion
In Stephen Cox book THE BEVERLY HILLBILLIES on page 19-20 its covered how the location to the mansion was leaked to TV GUIDE just before the film crew was going to go up and film color exteriors of the Kirkeby Mansion. Low brow idiots began showing up invading the grounds and even thinking Granny and co really live there so the owner promptly banned any new filming up there. The book says the public didn't even notice the loss of exterior shots but as a die hard fan of the show I immediatly noticed when I was young watching the reruns in the 1970's.
I think the mansion itself was almost another character in its grandeur and I really liked the episodes that went on location up there none so more than GRANNY'S GARDEN when she was going to plow up the front lawn.
Anyway back to the threat title. My theory was 50 years ago when there was no Google Earth, Internet what have you it was much easier to keep its location a secret.
In THE BORDER part of the plot has the Clampetts planting these signs along the way leading to the mansion. I feel that the road with its well manicured trees and tiered cement wall leading up a hill to the mansion would have been the visual clue someone trying to find the mansion would have needed to track down its location once and for all. If you watch the episode where the characters are in a car driving that road its very long and very distinctive. I bet whoever saw that either right after it aired or waiting again for the summer reruns decided to find the mansion and eventually leaked it to TV GUIDE.
Its a damn shame. The show felts sort of claustrophobic after that not ever getting to see the mansion in all its glory. Heck they even changed the trees on the mansion set since they no longer had to match it to the real house.