The residents, a couple in their 60s, managed to escape the house without injury and drive to the police station, Lt. Joe Wade of the Atherton Police Department said in a telephone interview.
Police responded to the scene in force and with a police dog, and set up a perimeter, but the robber was gone, Lt. Wade said. It's not clear where the burglar entered the house or how he got to the second floor, he said.
The residents described the man as slender, 18 to 25 years old, and wearing a black mask. The investigation continues, but with a special urgency: The burglar invaded an occupied dwelling.
"Everyone takes it more seriously when there's someone in the house," Lt. Wade said. "Property crimes happen all the time and people burglarize houses every day throughout the state," but burglars willing to enter an occupied house "are a much smaller group."
The incident unfolded as the husband was climbing the stairs after his wife told him she had heard a noise upstairs. While on the stairs, he met the burglar, who then grabbed the husband and demanded money, Lt. Wade said.
The wife made a run for it and had started the car when her husband ran out and joined her. He had bargained with the burglar by offering him $100 in cash, and escaped somehow, Lt. Wade said.
"As frightening as the situation was, they were both able to handle themselves very well," he said.
Near a fence in the yard, police found a laptop computer and a model car stolen from the house and lying in the grass. In an adjacent property, they found a knife and a pellet gun believed to have been used in the crime.