。
The jewels were found in an unexpected place.
Literal
Jewels [topic-は] unexpected place [at-で] were-found
思いがけない = unexpected (思い 'thought' + がけ from かける 'to hang/expect' + ない). 見つかる is the intransitive partner of 見つける — 'to be found' without specifying who found them, a common Japanese strategy for expressing passive-like meaning without the passive voice.