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During the events of The Heroes of Olympus, Gaea opened up the Doors of Death so that the monsters could come back easier.
However, I could find no mention of how they got back without the Doors. But we know that they come back, as the Furies and the Minotaur regenerate.
And if there was an easy way to get out of Tartarus, it wouldn't be such a great prison, and we see that it is used as a prison for the Titans.
How do the monsters get out of Tartarus normally?
The doors of death are the only popular pathway to Tartarus. Between both series, four entrances to Tartarus have been shown. One in the Underworld (main entrance), one in the Labyrinth (destroyed/sealed off), one underneath Rome, and one inside Epirus, the House of Hades (via the Doors of Death, which have now been removed). As you can see there are several pits or holes which lead to Tartarus. It is through these holes that the monsters come back.
Riordan Wikia also states
The map of Tartarus shows a door leading to the mortal world, which is presumably used by monsters after reforming.
Sources
Riordan Wikia
The Books
