I feel like this should be a fairly common plot device but for some reason I can't think of one example off the top of my head.

Here's the situation I'm looking for: A mystery/crime is considered "solved", but a person with intimate knowledge of the situation (like the detective) is still wresting with the outcome because something just doesn't sit right about it. So he's sitting at home pacing, or lying in bed thinking it over and suddenly BAM it hits him and he grabs his coat and heads out to expose the truth (i.e. the real killer).

The point in Breaking Bad at which Jesse figured out that Huell pickpocketed the poison off him is similar to this, but not quite right. Because in that case, Jesse was not intently thinking about what didn't add up. Rather something just came along and triggered a thought.

S1 of True Detective also has some similarities, except in that case the protagonists knew all along that their story of finding the real killer was somewhat fake.

Ideas? This should be common enough to be a borderline trope

This post was edited by Kayeto on Mar 30 2019 09:24am

The killing season 3?

Could be certain seasons of The Mentalist, Bones and that Writer / Detective show... forgot name.
Mind you, in my response it would most likely be none of those, but they have similarities as well.

Not sure if it fits your desired narrative but i instantly thought of the end of the Usual Suspects when Chazz Palminteri drops the coffee cup and the realization that happens afterwards (Keyser Soze etc).

Happens in the mentalist sort of

Other than that and true detective s1 idk really you’re right it should be easier to think of lol