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속 does actually mean "inside"!
속이다 (a verb), however means "to decieve".
Removing -다 and adding the verb ending -ㅂ니다 turns the verb in its "high formal" form 속입니다, which is the respectful way to say it.
To say "She's inside", you can say "그녀는 속에 있습니다" using the location marking particle 에 and the verb 있다 which means "to exist".
You can also use 안 to mean "inside" instead of 속.