The vehicle was parked behind the Slaughter House next to the grass bank on Castle Pier, St Peter Port between 22:00 on Sunday 21 March and 11:10 the following morning.
Guernsey Police is now seeking witnesses or asking anyone with any information to contact them.
Anyone who can help the investigation is asked to contact PC 73 SARGENT on 222 222 or to contact CrimeStoppers anonymously on 0800 555 111.