Officers were called to the Westmont retirement community at 1675 Scott Blvd. after a stranger came up to a female employee around 9 a.m. and told her to "stay away," and not to follow him, manager Guy Cox said.
The employee said she saw a gun tucked into his pants, Cox said.
Residents were evacuated from the building and waited outside on the sidewalk for more than two hours, Lt. Wahid Kazem said.
The lieutenant said that officers searched all the floors and rooms, but didn't find anyone matching the description given by the employee.
No firearms were found in the building, he said.
The residents were allowed to return to the building just after 11 a.m.
"Everybody is fine," Cox said. He said the average age of the residents at Westmont is 85.
Kazem said the suspect was "vaguely" described as a man in his early to mid-20s. The employee became concerned when he approached her and asked for the manager by name, Kazem said.
The employee lost sight of the man after the interaction, and police have no leads as to where he may have gone.
Staff at the retirement center were instructed to call police about any further suspicious activity, he said.