What does the term "unwitting possession" mean and is it a valid defense?
Unwitting possession means that you possessed a contraband substance without knowing you had it. The classic case is mail carriers - they deliver a package containing cocaine but are not guilty of possessing cocaine because they didn't know (and had no reason to know) that cocaine was in the package. The other classic case is borrowing someone's car and the owner had cocaine stashed in the door frame. Some states allow unwitting possession as a defense, some add the 'had no reason to know' element, and some don't specifically recognize the defense at all.
However, in all jurisdictions, evidence which convincingly proves that the defendant did not know the drugs were there eliminates the element of “mens rea,” or “guilty knowledge or intent.” It is not a crime to be unlucky. The defendant usually has the burden of proving unwitting possession by a preponderance of the evidence. Look at it this way, if a jury is faced only with evidence that there were drugs hidden in a car used by multiple people and no evidence at all that the one arrested is the one who put it there, or even uses drugs, it is difficult to imagine how such a jury could find “guilt beyond a reasonable doubt.” The missing other drivers of the car are as likely to be the ones who put the drugs there as the one arrested. The more deeply hidden the drugs are, the less likely the driver knew they were there.