Marquardt, 45, a self-proclaimed "ghost hunter," appeared on Australia's "The Morning Show" on Channel 7 and talked about her sighting.
The former model said she did not get a good look at the spirit, but claimed that "it looked like a woman standing there," according to the Daily Mail.
She also said she felt a "positive vibe" from the female spirit, according to the report.
"I didn't get a negative vibe at all," she said. "I got a very positive vibe and I think that it might have been a former employee of Hef's just coming to see the new addition to the family."
The Holmby Hills estate was built in 1927, featuring 29 rooms, a wine cellar and a grand hall, the report said. Hefner lived there for 43 years before his death in 2017.