The couple told officers that they returned home from a vacation Sunday afternoon and found the newborn in their backyard.
Folsom police Sgt. Chris Emery would not say whether the baby was found clothed or wrapped or where in the yard it was found.
He said the baby did not have any visible signs of trauma on its body.
He said he did not know the baby's gender.
On Monday morning several people visited the home, but they declined to comment.
Emery said he does not know whether the couple, the home, or anyone the couple knows is connected to the baby.
Neighbors said that there was no indication there had been strangers in the neighborhood - no dogs barking and no one who looked suspicious.
Several neighbors said they were worried that the couple who reported the discovery may be traumatized and that the baby's parents may be distraught.
"I just hope that they would get help," said Diane Kim, referring to whomever left the child. "You know, obviously, they need some kind of help. I hope they find some kind of peace with whatever they went through."
Emery said that Folsom police detectives are hoping that anyone with information about the baby will call them.
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