In season 5 of Dexter, Detective Joey Quinn is heavily suspicious of Dexter and what he gets up to. Joey then hires Liddy to spy on him. Liddy gets some good evidence including pictures of Dexter and Lumen going out on his boat to dispose of a body. Dexter kills Liddy and its fairly obvious to Quinn that Dexter must have done it.
Blood is discovered on Quinn he gets brought in to be questioned. Quinn eventually gets released, but basically gives up in his pursuit of Dexter. Why is this? Is it just because he doesn't want to upset Debra? (I'm on season 6, so I'm unsure if this issue resurfaces in later episodes.)