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Depends on what you mean by "Not found on a map".
If the place is "somewhere in the south pacific" or something similarly vague but theoretically known, it'd be Where the Hell Is Springfield?.
If a place is not on the map because it can't be mapped (It has a spotty connection to reality, it moves around, it's invisible to satellites, etc) that's more in the realm of tropes like Pocket Dimension, Vanishing Village, Just One Second Out of Sync, and Lost World.

As you might have noticed, when comics and movies take place in fictional countries, we usually don't get to see on a map exactly where in the Arab world Qurac is to be found, or where in Eastern Europe Ruritania is located. Do we have a trope for this?
Edited by MichaelKatsuro