In the US there were many cases of free blacks being kidnapped into slavery because of the way blacks were treated differently than whites. You are right, there were very few pieces of paper saying "This man is free". Some manumissions did involve such a thing, but slave catchers rarely cared about that.

Many states had Negro Exclusion Laws, and most states made it illegal for a black man to testify in a case involving a white man. Meaning If a white man accused John Q Public of being a slave, the only valid testimony would come from white men. Outside of the bravest abolitionists, these testimonies were hard to come by.