The thief is definitely somewhere in here. There must be a hideout — all we have to do is find it.
Literal
Thief [topic-は], without-doubt here [at-に] exist [should-はずよ]. Surely, somewhere [at-に] hideout [subject-が] exists [because-から], there [object-を] finding [only-さえすればいいの].
間違いなく ('without a doubt') and きっと ('surely') double down on certainty. 根城 (ねじろ, 'hideout, base of operations') is originally a castle term. ~さえすればいい ('all you have to do is') is the 'if only/just X' pattern. はずよ uses はず (expectation) + feminine よ. の at the end is the feminine explanatory. A detective/mystery scene with a female speaker confidently directing the search.