A wizard can copy a spell they find into their spellbook. This is described in the "Your Spellbook" section of the Wizard’s class features:
When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a spell level you can prepare and if you can spare the time to decipher and copy it.
Notably, it does not say "when you find a wizard spell in a spellbook". Are there any instances in published adventures where a wizard can copy a spell from something other than a spellbook?
I’m obviously not concerned about spell scrolls here. I’m looking for something like a spell written on a wall or stone tablet, or other surface that does not require a check like a spell scroll does.
This Q&A firmly establishes that the wizard can copy their spells from any written source, but I am not aware of any published examples of this outside of found spellbooks.