Police found a knife with a six inch blade underneath a car passenger seat when a man left his BMW parked on double lines with its engine running.
A police officer at first reached inside the car to prevent the car being stolen, but then spotted a female in the passenger seat who explained the driver, Ryan Fewkes, had just gone into the shop.
Rebecca Ritson, prosecuting, said the officer could smell cannabis inside the vehicle, searched the BMW and recovered a knife from under the passenger seat.
Fewkes (24), of Sandringham Walk, Spalding, pleaded guilty to having a knife in a public place, Commercial Road, Spalding, on January 16 when he appeared before town magistrates on Thursday.
Sentence was adjourned to February 19 for a probation report.
Miss Ritson said Fewkes told police a friend had placed the knife under the car seat a few days before.
Fewkes said he meant to remove it himself but had forgotten to do so.
Solicitor Mike Alexander, mitigating, confirmed the knife belonged to one of Fewkes’s friends, who had been using it and left it under the seat.
Fewkes was already on a community order a further order might be a consideration.