Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
unmarked [ʌnˈmɑːkt] adj (unstained) чи́стый*чист
unmarked police car полице́йская маши́на без опознава́тельных зна́ков
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.