Jessica,
Try closing your eyes and look at a few of the female visitors who are sitting funny in the visiting room. They are carrying drugs in a place where the sun doesn’t shine or the place next to that. It would have to be put somewhere inside the visitor’s room where it can be retrieved later by a inmate trustee or another person on the inside.
Sad to say but it could anyone who works in the prison, including mental health counselors like those arrested not too long ago. 
Maybe....a drone delivered via air drop into a recreation area outside!