If you've gotten to the estate, you should have had to do the greyhaven questline where you sneak out of the inn to go to the Estate for the first time. I believe that is the requirement to finding Knave in the Forest - mainly by exploring the Forest. After that, he should be a person to find within the Forest! If this isn't the case, then this is a bug report - In which case, I'll look into why this is happening!