Do you know the difference between a red herring & a plot hole? Red herrings, usually used in mysteries, distract suspicion from the true villain and point toward other possible culprits. These can also be used in similar situations to keep your readers guessing.
A plot hole is something implied that you never brought to closure. While you may plan on doing so in a sequel, it’s best to nonetheless mention it in some way so your readers don’t think you forgot and it slipped through the cracks.
When you write a murder mystery, you must keep your readers guessing. Any “whodunnit” story needs red herrings to place doubt in the reader’s mind regarding who the culprit might be. No matter how many novels your readers have experienced, they shouldn’t be able to easily predict how it will end. Readers thrive on suspense and wondering what will happen next.
These red herrings may necessitate a few characters who are technically extraneous. These, of course, are the exception to the rule to not include people with no function in your story. The fact of the matter is that they DO serve a function, and that is to keep the reader guessing.
Marcha's Two-Cents Worth 