RELATED SLIDESHOW: Inside the Garrido home
The couple is charged with the 1991 kidnapping of Jaycee Dugard.
Their Antioch home has become a focal point of investigators reviewing outstanding kidnapping cases in the San Francisco Bay area.
Nelson is not saying where in the yard the dogs picked up a scent. He also cautioned about false positives.
Investigators plan to scan the site Friday with high-tech equipment.