This is the 13852nd most frequent Polish word.
policyjnych
"Policyjnych" means "police" (adjective, relating to police) in English.
The word 'policyjnych' here is used as an adjectival form describing the radiowozów (police cars), attributing them as pertaining to the police.
Na ulicy widziałem kilka radiowozów policyjnych.
On the street, I saw several police cars.
In this sentence, 'policyjnych' functions as an adjective modifying 'syren' (sirens), pointing out that the sirens are associated with the police.
W nocy słyszałem dźwięk syren policyjnych.
At night, I heard the sound of police sirens.
Here, 'policyjnych' describes 'technik operacyjnych' (operational techniques), indicating that these techniques are related to police operations.