The Warden can appear in a vision while you are sleeping

Minecraft helps players discover parts of the game by providing clues such as explorer maps that take you on a journey to Woodland Mansions, or the basement under igloos implying you can cure Zombie Villagers, or ruined portals to guide you to the Nether. There could be something similar for the Deep Dark.

The Deep Dark is like the mysterious underbelly of the Overworld. Its secluded deep underground, out of the way and easily ignored. The Warden lives under the Overworld like a monster under your bed. When you get visions of the Warden in your sleep, its a hint that theres something sinister in the world. And for those who've met the Warden, its a reminder.

Normally when you sleep, you look down at yourself in bed as everything fades before waking up. There would be a very rare chance that your surroundings turn into an empty void, all you can see is your body and your bed while everything else fades to black. There will be nothing except for a shadowy figure in the dark: the Warden. The Warden's pose and placement is random, it could be looming over your bed or standing in the distance. Its glowing chest beating would be last thing you see before you wake up.

By the time you notice it, the sleeping animation would be done and you'd be awake again. It doesnt affect anyone else so It could happen to you in multiplayer. It'd leave players wondering what they saw and what it meant. If they ever wander into the Deep Dark, maybe they'd remember seeing that place in their dreams.