The guard pulled the elderly man aside and went through his bag. Inside, he found a 1.95-franc packet of grated cheese lodged between the very bottom of the bag itself and its cardboard lining.
The packet of cheese, which was squashed flat, did not feature on the man's shopping receipt.
The shopper was subsequently fined 150 Swiss francs for shoplifting and made to fill in a couple of forms. He was also banned from entering any Denner store in Switzerland for two years.
In a report on the incident, the security guard spoke of the man's bag as having a false bottom, specifically for the purpose of theft.
The former government worker, now 85, says he had no idea how the cheese came to be in his bag on that day, but believes it may have gone unnoticed after an earlier trip.
The story should have ended there. However, the shopper allegedly broke the ban on entering Denner stores (the firm is owned by supermarket giant Migros) two months later, according to a detailed report of the case in Swiss regional daily the Aarguer Zeitung.
In the second incident, the pensioner was at another shopping mall in Aarau when he paused briefly to look at the fruit and vegetable section outside the front of the Denner store there. However, he did not enter the shop as he knew he was barred from doing so.
But as the man walked on, he spotted the same security guard who had found him guilty of shoplifting two months earlier.
Three months after this second incident, the shopper received a legal complaint from state prosecutors and filed by a representative of the security company hired by Denner alleging he had unlawfully entered the premises of its store by stepping off the beige flooring of the shopping mall and onto the grey flooring belonging to the supermarket.
He was hit with a 500-franc fine, which would only have to be paid if he re-offended, but was also forced to pay costs of 1,000 francs, according to the Blick newspaper.
The man appealed to the district court and saw charges and costs dropped, with the court ruling that video evidence was inconclusive and that the security staff who had lodged the complaint did not have the authority to do so.
But cantonal prosecutors in Aargau disagreed and the case was tried again in a higher court. In November 2017, the pensioner was again found guilty. The fine of 500 francs was reduced to 150 francs but the man ended up paying charges and costs of 2,500 francs.
The shopper told the Aarguer Zeitung he had spent around 10,000 francs on the case to date but did not have heart to take it any further.
On Thursday, Denner told the Blick newspaper the security firm in question was no longer used by the firm.
A spokesperson for the company owned by Migros apologised and said the firm would be in touch with the shopper and pay all costs.
"It is difficult to understand what took place two years ago," the spokesperson said.
In a Facebook post, Denner said it was investigating the incident and noted the firm had not been a party in the legal case.