Martin Vasic, 29, of 1, Vauvert Terrace, Vauvert, St Peter Port, denied two counts of failing to comply with the requirement to self-isolate.
He was found guilty of one but acquitted of the other and fined £3,000.
When giving his decision, Judge Gary Perry was critical of how Law Enforcement officers appeared to have given various answers about the date on which Vasic would be free from isolation.
The court heard how the defendant had returned to Guernsey on a flight from Southampton on 24 July. It was explained to him at Guernsey Airport that he would need to self-isolate until 7 August.
CCTV footage showed a man, which the prosecution said was Vasic, walking through Town early in the morning of 30 July.
Vasic denied it was him, but Lewis Carroll, a chef at the White Rock Cafe at the time, said the defendant had entered the cafe at between 6.30 and 7am that day.
Later that day, Law Enforcement officers went to Vasic’s home for a welfare check. It was the prosecution case that clothing in the room, which included the shorts Mr Vasic was wearing when he answered the door, matched that of the man seen on the CCTV.
Body-worn camera footage showed Vasic asking the officers when his self-isolation period ended. One officer told him it was 7 August, while another said 10 August.
At about 6pm that same day, two police officers visited Vasic to perform another check. He did not answer the door and contact was later made with him through a third-floor window.
He told the court that the constable had said he could leave the house on 5 August, which another said he did not recall, but could not rule out.
Vasic was again spotted on CCTV in Market Square on 5 August. He accepted it was him but said he had gone out only because he had been told he could in the earlier discussion with police.
He argued that was a reasonable excuse, while he claimed the earlier incident was a case of mistaken identity.
Judge Perry agreed that images of the defendant and his clothes matched.
He said Mr Carroll’s evidence had been compelling and he found Vasic guilty of the first offence, but acquitted him on the second charge because he had been given three different release dates by Law Enforcement officers and it would be natural for the last-mentioned date to be in his mind.
Judge Perry said the courts treated self-isolation breaches very seriously.