They were under ATC. ATC can track objects in the air, even if they're not using a transponder. Using primary radar, ATC will be able to provide traffic advisories. Police helicopters usually fly under "flight following", meaning they would like to be informed of other traffic.
Oh, noes! You KNOW that isn't true!
Don'tcha remember?!? On 9/11/01., all the "Hijacked" Airliners simply DISAPPEARED from RADAR because they switched their Transponders off... (Rolls eyes) [/sarcasm]