It's not illegal for cops to sit with their lights off at night.
Cops can legally sit with their lights off at night and hide to observe traffic and catch people violating traffic laws.
The cop can legally hide to observe any traffic violations and the cop doesn't need to announce their presence before they initiate the stop.
This is not considered entrapment, as entrapment involves the police inducing you to commit a crime that you wouldn't have otherwise.
Most times though the police may hide without lights and only light you up when they get behind you.
If the police car is not marked, you can call 911 and let them know where you're at and confirm that it's an actual police officer pulling you over.
Cops also don't have to have their headlights on at night and cops can also legally hide with their headlights off and they can pull you over for violating laws, even though they are doing the same thing.
As long as the cop or police have reasonable suspicion of some regulatory violation or traffic violation, like a blown headlight or taillights, turn signal lights etc being out, the cop or police can pull you over.
The initial stop is often considered a low level stop, but it also gives the cop, the opportunity to look for other more serious violations like open containers or expired registration.
The officer also has discretion in how they handle the traffic stop, and it often comes down to the officer's judgement at the moment.