Persistent bad breath despite diligent oral hygiene can be perplexing and frustrating. You've already considered many common causes. An area that some people tend to overlook, and could be causing your problem, is the area under the nose - yes, sometimes the due to accumulated sweat and oil. That being said, it's also possible that the bad smell originates from inside your mouth, despite your meticulous oral care routine. It's important to remember that food particles caught between teeth and under gums can be broken down by saliva bacteria, leading to putrid-smelling gases that can be mistaken for a bad smell coming from the lips. If you've explored all these possibilities and still encounter the issue, it might be time to seek medical advice. Certain systemic issues, including infections or stomach problems, can also contribute to bad breath and might require professional intervention to diagnose and treat​​.