https://www.metv.com/stories/the-beverly-hillbillies-mansion-address-was-leaked-and-fans-caused-a-little-trouble

Mrs. Kirkeby, the mansion's owner, was mourning the loss of her husband, and fans would knock on the door and ask, "Where are the Beverly Hillbillies?"

In another thread I speculated the location was likely determined by the episode where the Clampett's get a butler. In the episode Drysdale and Hathaway are driving to the mansion and some very distinct looking trees could be seen leading up to it. Some enterprising person at TV Guide was probably able to track it down by using those trees as a reference. Just my guess.