。
I left my passport somewhere.
Literal
Passport [object-を] somewhere [at-に] left-behind.
置き忘れる is a compound verb: 置く (to put) + 忘れる (to forget) = 'to leave behind accidentally.' どこかに (somewhere) uses か for indefiniteness. 旅券 is the formal word for passport — more common in official contexts than パスポート.