The man, described as a white male adult in his late 40s, walked into the child care center at the Livermore Valley Tennis Club around 12:15 p.m. today, according to police.
He told an employee that he was there to pick up his son, and pointed at a 4-year-old boy in the center.
When the employee asked him for the standard verification required by the center for anyone picking up children, however, the man was unable to produce and left the center abruptly.
The boy's mother was on site when the incident occurred, told center employees that she had not authorized anyone to pick up the boy, at which point the childcare center contacted police.
Police were unable to locate the suspect in the area, but have released a sketch created with the help of a witness. He is described as being around 6 feet tall and 250 pounds with a large build. He had short blond hair with long sideburns and was wearing a green baseball cap, a white and gray shirt and khaki pants.
Anyone with information on this suspect should contact Livermore police immediately.