There would need to be a reason why her mother wasn't taken into the sanctuary and so she could be born and raised there.  Maybe her mom was a rebel that refused to go?  Maybe her mom was exiled when she was born?  Maybe only one of her parents was from the sanctuary?  Maybe her mom took too long to go underground?

Basically, Metal-doctor keeps biometric readings of all members of the Threestbway on file from birth, so only they can use the elevator (also gives newborns medical nanites, but that doesn't matter here).  If you were born outside, no one else would know how to add it, so you'd be stuck outside forever.

You could also be from another tribe.

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This Sanctuary was originally USAF property, and they brought in any civilians that made it into the subway after the bombs, but the security system and access to its systems is based on rank, which everyone inside is given based on their value to the sanctuary.  Babies and children are civilian contractor/non-coms, and adults get ranks.