A group of young people were found with a machete in a disturbance in a residential street.
Sussex Police say they recovered the knife in Hawthorn Road involving a "number of young people".
Officers are now looking to identify a passerby who they say picked up the knife at the time while walking her dog.
A spokesman for Sussex Police said: "A woman walking a dog was seen to pick up the knife, and put it back down.
"Officers are keen to speak to her regarding the incident and believe she may help with enquiries.
"She is described as in her 40s, with blonde hair held back in a clip. She was wearing a Puffa style jacket with a fur hood and walking a whippet."
The incident is said to have taken place at 5pm on October 14.
Anyone with information is urged to contact Sussex Police, quoting reference number 47240200727.
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