Book #19, The Quest of the Missing Map , where Nancy searches for a treasure hidden by a seafaring grandfather.
If you were thinking of a different series, "The Missing Map" is also:
The episode features a rare sequence where Michelangelo walks the boys through a convoluted path to ensure they can't easily find the lair again—showing the Turtles actually do care about security. [S3E40] The Missing Map
The episode "The Lost Mine" (often associated with missing maps or documents) where Poirot investigates a murder linked to a missing mine map.
The episode serves as a heavy-handed moral story about the dangers of theft and envy, with Walt eventually learning his lesson after putting both his brother and the Turtles in grave danger. Fun Facts & Trivia Book #19, The Quest of the Missing Map
Fans often point out the plot hole of why the Turtles would even keep a physical map to their own home if they wanted to keep it secret.
A Season 3 episode where Map is accidentally taken away in a package during a game of hide-and-seek. The episode serves as a heavy-handed moral story
Walt, envious of the attention his brother gets from the Turtles, steals a rare pouch containing the only physical map to the Turtles' secret lair.