The note reads: "stay over tonight after the game?"
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"Pretend this note has something concerning."
I have players in my group that would read that out aloud, and then go "OOOOH!"
Boutta find out why they call me El Dr. Itch!
Isn't it just Detect Magic or am I missing something?
I did a search and found this which I suspect is what they are referring to.
You gain the ability to see into the hidden layers of reality, revealing eldritch traces and secrets. For the duration, you gain advantage on Intelligence (Arcana) and Wisdom (Insight) checks.
Additionally, you can see creatures that are invisible, ethereal, or otherwise hidden within 60 feet of you, though you can’t perceive exact details. Creatures detected in this way appear as faint, shadowy figures in your vision.
That site is so annoying because it's all homebrew/third party but gives the impression it's official content.
Maybe they're thinking of witch sight.
Witch Sight
Source: Player's Handbook
Prerequisite: 15th level
You can see the true form of any shapechanger or creature concealed by illusion or transmutation magic while the creature is within 30 feet of you and within line of sight.
Or more likely, it's a DnD meme, they probably didn't read the description.