The Department of Public Safety received a call at about 9 p.m. of a package on the fourth floor of the garage located at 121 W. Evelyn Ave., according to city spokesman John Pilger.
Public Safety officers arrived and confirmed the location of the suspicious package, shut down the garage and contacted the San Mateo and Santa Clara County sheriff's offices for support.
Shortly after midnight, a Santa Clara county bomb squad determined the package was not an immediate threat and allowed people with cars in the garage to be escorted to their vehicles to exit the garage, Pilger said.
"Once we determined that there didn't appear to be an immediate threat or explosion, you breathe a little sigh of relief because it's not the type of situation where someone's going to get hurt," Pilger said.
An X-ray of the suspicious package, a cylindrical canister about 12 to 16 inches long, showed that there was liquid inside, according to Pilger, but authorities have not determined the type of liquid yet.
A private contractor from Fremont is on their way in to the garage to examine the canister and package it for transport to a safe holding area, Pilger said.
The garage is reopened except for a small area cordoned off on the fourth floor, where authorities hope to remove the canister by about 4 a.m., Pilger said.
Caltrain routes that travel through Sunnyvale were shut down Wednesday night by the San Mateo County Sheriff's Office as a precaution, but Pilger said that the incident should have no impact on morning commutes by the time Caltrain routes resume at 4:30 a.m.