If they are not sleeping (and so, they are wandering some area and you're told to find them), you really just have to wander around until you bump into them. Catching them between hotspots will be hard, but all you need to really do is visit all the hotspots to have the best chance of finding them. I would recommend checking the cafeteria, prostitution spot, stocks, and Underground - an inmate spends most of their time in these areas, when not interacting with other inmates.