Sussex Police has issued a warning to holidaymakers after making the discovery in boggy field near the airport on Wednesday June 22.
Officers discovered hundreds of keys to the vehicles in the boot of a car which was left open and unlocked, while several others were covered in mud.
In addition, some keys were left in envelopes on the windscreens of the cars they belonged to, in full view of anyone passing by.
The force said it is working in conjunction with Horsham District Council and West Sussex County Council Trading Standards to investigate five companies suspected of operating illegally at the site.
Police are urging drivers to beware of rogue traders and only book with approved parking companies, pending the investigation.
The location of the field is not being released to protect customers’ vehicles, officers added.
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