The most common form of chemical testing used during a DUI stop is a breathalyzer, which measures blood-alcohol levels through breath and is unreliable.
A surprising number of things can cause a false positive for a breath test.
Over-the-counter medications
Many over-the-counter medications, such as Nyquil and many Vicks products used to treat common symptoms of cold and sinus, contain alcohol. Even some cough drops have alcohol in them. You would need to take a significant amount for it to show up in your blood, but if you just took a dose of Nyquil cough syrup, you could test positive without having a single drink before driving.
Acid reflux
Anyone who suffers from acid reflux knows that it is a painful condition during which acids from the stomach move up into the esophagus and mouth. What they may not know is that it can also cause a false positive on a breathalyzer test.
Ketosis
If you know of the popular keto diet, you may know that it causes your body to burn fat rather than glucose for energy. It is also a common condition for people with diabetes. The overproduction of ketones during ketosis can cause acidic breath, which can also skew an alcohol breath test.
Breath enhancers
Mouthwash and breath sprays contain alcohol. If you just used either, the alcohol could easily produce a positive breath test. Even a strong breath mint could do the same.
To be safe, most attorneys recommend declining a breath test if pulled over on suspicion of DUI.
