Imperial Oil's sirens sounded at the Sarnia site around 3:15 p.m. on Monday afternoon.
The company says personnel quickly responded to a report of a fire in the refinery but when responders arrived, no fire was found.
As a precaution, the site sirens were sounded and a CVECO Code 8 was activated.
Imperial says an all-clear was issued within minutes, and no injuries were reported.
The company is apologizing for any inconvenience the sirens may have caused and continues to investigate.