The guard, Lloyd Collins of Oakland, called police about 12:15 a.m. to report someone causing a disturbance in the 400 block of Southwest Boulevard, according to the Rohnert Park Department of Public Safety.
Collins, 33, told police a person was threatening him and while they were investigating the scene, one officer noticed what appeared to be cocaine on a tray, on the passenger seat inside Collins car.
Collins was detained and during a search of his car, the suspected cocaine and additional narcotics were seized, police said.
A loaded handgun was also found by police inside the center console of Collins car, along with two loaded magazines for the gun, police said.
The gun wasn't registered to Collins and he did not have a permit to carry it, police said.
Collins was arrested on suspicion of possession of the suspected narcotics, possession of a loaded firearm while possessing narcotics, and possessing a loaded concealed firearm.
Collins was booked into the Sonoma County Jail where he is being held on $10,000 bail.
The person who had been reported as suspicious by Collins was released after the officers concluded their investigation.