Why does it seem like in every cop based TV drama (CSI, Law and Order, etc.) that whenever something bad has happened that the front door is always left open or slightly ajar? Do criminals not realize that this is a tip for cops to know something is wrong?
TvTropes calls this the OminouslyOpenDoor and files it as a death trope, meaning that usually the investigator will find a dead suspect or witness in the room:
There are two sides to this:
In-universe explanation aka: Why would they leave the door open?
Of course this does not always make sense and quite often this is simply illogical behaviour from the bad guys.
However there are some valid reasons why the door would not be shut:
Analytical point of view aka: What is the purpose of the scene?
Even if there is a valid plot explanation for the scene, this is still mainly used as a device by the director to build up tension. The investigator usually arrives at the door with the motive to speak to someone. Sometimes this happens at his own door when he comes home.
Before the ajar door is noticed, normally there is little to no suspense. Often two characters will be talking casually until they see the door.
Then suddenly: Excitement!
Something is wrong - this door should not be open! The viewer gets drawn into the action and knows that a surprise must be lurking behind this door. So basically the door acts as a gateway between different atmospheres and is used to quickly change the pace of the episode.