This is the 13688th most frequent Russian word.
неопознанный
Unidentified.
Here, "неопознанный" describes an object that has not been identified or recognized.
На горизонте появился неопознанный объект.
An unidentified object appeared on the horizon.
In this sentence, "неопознанные" refers to documents whose origin or content has not been determined or understood.
В архиве нашли неопознанные документы.
Unidentified documents were found in the archive.
"Неопознанный" here applies to an item (a car) that the police have not identified or associated with a specific case.