Guernsey Police say it happened “sometime between 16:45 and 17:15” on Monday 22 April.
Officers are now looking for any witnesses, particularly those who may have CCTV of the incident.
Anyone who may have any information which can help the enquiry is asked to contact PC66 Smith at Police Headquarters on 222 222 or to contact Crimestoppers anonymously, on 0800 555 111.