Illinois was part of the Northwest Territory that was designed from its beginning not to have slaves. Illinois did at one time try to end this law after they became a state, but it was over turned. Persons of color were always counted in the census and the census data was not made available to the public beyond the gross counts, so the person and their location was hidden. Illinois was a place where if you were that far north it would have been difficult to snatch someone and get them back to slavery with out proof or ownership and arrest for kidnapping if they couldn't provide papers showing ownership. Just the same, being cautious was probably a good idea.