There is some confusion over whether something can be categorized as a secret, easter egg, or myth in the Grand Theft Auto series. All info about the differences can be read here.

A secret is a part of the game that is usually not intended to be accessed. For example, the buildings where cutscenes are animated often aren't normally accessible in-game, but they can be accessed through 3rd party trainers. The Andromada plane in Grand Theft Auto: San Andreas is a real vehicle used in a cutscene but is never made available to the player, however, there is a secret way to obtain it.