It was announced a few episodes ago that Officer Tamin (Kat) passed her exam and is being promoted to the rank of Detective 3rd grade. Her promotion ceremony has not yet been shown, but it is inferred that it happened already as she is now introducing herself (and being introduced by others) as Detective Tamin. Yet she is still carrying her silver shield, if she has indeed been promoted to the rank of detective she should be carrying a gold shield. However if her promotion ceremony has not yet happened it would be improper NYPD procedure for her to be addressed as detective until she is given her detective's shield at said ceremony.