Cops will flash their lights but not pull you over as a warning or when the cop is on their way to another call that is more important and they cannot stop you but want to warn you.
The cop flashing their lights at you but not pulling you over is a visual warning and most cops have 12 hour shifts and stops take a lot of paperwork write up time and it's also likely the cop was ending his shift and the violation was not serious enough that he wanted to pull you over.
The reason why cops drive with their blue lights on but not flashing is for visibility and deterrence.
The practice of cops driving with their blue lights on but not flashing is called using "cruise lights".
The blue lights that cops drive with or "cruise lights" indicate an active police presence, without requiring drivers to yield or pull over as it does not signify an emergency or traffic stop.
The method of using cruise lights and driving with the blue lights on helps deter crime and makes the officers more visible, especially in areas that have high foot traffic or during night patrols.
Also using the steady blue lights instead of the flashing blue lights can also prevent a situation from escalating, like when assisting a stranded driver or responding to a call that might be sensitive.
The blue cruise lights are used by police in situations where an officer is on a low priority call or are just patrolling and is a way to show their presence without needing a full emergency response with the flashing lights and sirens.
If a cop is behind you and they have their steady blue lights on, you don't need to pull over or get out of the way because it's not an emergency and they are not pulling you over.
However if the cops blue lights start flashing and or you hear a siren you should pull over, because either they are trying to pull you over or they are needing you out of the way so they can respond to an emergency call.
Cops can also 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.