A woman found the word cow scratched on her car when she returned to the vehicle.
Police are linking it to a similar attack earlier this year in which the victim's car was so badly damaged by graffiti it was written off.
They believe she is a victim of harassment and want to trace witnesses to the latest attack.
The woman, who has not been named, left her vehicle in a car park in Chandos Road, Worthing, on September 5 between 3.30pm and 6pm.
She saw graffiti had been etched into the paintwork with a key or similar object. The damage to the R-registered Suzuki was estimated at £900.
Inspector Chris Drew, of Shoreham police, said: "We are treating this as a case of harassment and criminal damage.
"The lady concerned was the victim of a similar attack outside her home in Shoreham Beach in May this year.
"We would like to hear from anyone who witnessed the attack in the car park near Bhs in Worthing."
Anyone with information is asked to phone Shoreham police on 0845 6070999.
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