He crossed the street to the dark office building and went through the revolving door. He left his sunglasses on. Inside the lobby, he spotted the building security guard, who was in his fifties and sat behind a check-in desk near the elevators. The large man had his suit jacket draped on a chair behind him and drank Diet Coke straight out of the can. He had the Chronicle puzzle page open on the desk, with half the crossword completed in pencil.

Frost put his badge in front of the man’s face and introduced himself.

“Is there a problem, Inspector?”

Frost showed him a photograph of Lucy. “Have you seen this woman? She’s missing and at risk. We need to find her.”

The man squinted at the picture through his reading glasses. “I don’t think so. Not while I’ve been here.”

“One of the street performers in the square told me he saw this woman enter the building,” Frost snapped. He used his cop’s don’t-screw-with-me tone of voice. “He was dead sure it was her, and she never came out.”

“Well, I don’t know. I guess I could be wrong.”

“The woman is a patient of Dr. Francesca Stein,” Frost said.

“Oh. Sure. Her office is on the top floor.”

“Then let’s get up there. Now.”

The guard eyed the phone in front of him. “I should probably call somebody.”

“Call whoever you want after you let me in there. This woman is in imminent danger.”

Frost marched to the elevator bank without waiting for an answer. He heard the guard’s chair scraping on the marble floor and then the tap, tap of the man’s leather shoes as he ran to catch up to him. The guard breathed heavily and stabbed the elevator button. The two of them got inside the car and rode in silence. When the doors opened at the top of the building, he let the man lead the way to the far end of the hallway. Double wooden doors led to Francesca Stein’s office, and her name was on a brass plate on the wall.

The guard swiped a passkey against the lock. As he reached for the door handle, Frost stepped in front of him.

“You can go back downstairs now,” Frost said.

“The rules say I need to go in with you.”

“I can’t be responsible for your safety.”

The guard studied Frost’s eyes, which were hidden behind sunglasses. He looked as if he might gin up the courage to question him, but Frost slid his service pistol from the holster inside his jacket as he inched the door open. Seeing the gun, the man beat a quick retreat back to the elevators.