This is the 472nd most frequent Czech word.
policejní
Relating to police.
Here, 'policejní' is used as an adjective describing the car related to or belonging to the police force.
Policejní auto projelo ulicí.
A police car drove through the street.
In this case, 'policejní' modifies 'sirens' to specify that the sirens are of the police.
Slyšel jsem policejní sirény.
I heard police sirens.
'Policejní' here describes the station, indicating it is operated by the police.