The incident happened about 18:15 on Thursday 14 November in the undercover area near to the entrance.
A man and woman, in their late 40’s, were involved.
Officers said they “may have been noted by anyone leaving or entering the Locale or persons using the cashpoint”.
Any witnesses who saw the incident, at the Co-op Longstore Locale, Le Bouet, should contact Guernsey Police on 725111 or Crimestoppers anonymously on 0800 555 111.
Pictured: The Co-op Longstore Locale.