In 2020, we shared an article from CD-i member The Mundy Sea found an unused audio track in the code of Zelda's Adventure, next to the 'unused sprite' discovered by CD-i member Shikotei when exploring the extensive maps and graphics of the same game.
Later, The Mundy Sea continued to find how the easter egg was activated and he managed to do it, as is seen in this video. The Mundy Sea explains: "Everyone... I did it. I figured out how to activate the Food Dude easter egg in Zelda's Adventure! It took 25 years, but it's here. Head to the area seen in the video, select the Diamond item, and hold down the use button for 6.5 seconds. He will appear from the trees."
From the earlier article on Interactive Dreams: "This unusual sprite was discovered by Shikotei in June while he was ripping sprites and maps from Zelda's Adventure. The sprite is loaded near the lake in the Forest of Canvula, but he never shows up while playing. Due to where he was found and what he was standing on, he was initially believed to be a surfer. "A couple of weeks later, I was researching Adventure's many, many unused voice lines for a TCRF page. One of these lines caught my attention - it featured a man saying "Radical, dude! Totally!". according to researcher The Mundy Sea. After discovering the line, I put two and two together and came to the conclusion that the unused surfer said it, and that he was an easter egg that hadn't been discovered until now."
[Thanks, The Mundy Sea]