I would assume that a viable population will be established when a few more straggle into Illinois and locate habitat that makes them want to stick around. That's what happened in Iowa. The things are pretty elusive even in areas where they are (comparatively speaking) plentiful. If they are anything like bobcats, you can get the impression that there are few if any of them even when the area is actually full of them. Of course, the big difference is that the panther kills of deer and livestock will be much more apparent, unless they end up hanging out in the middle of the Shawnee Hills or Mississippi and Wabash river bottoms where few folks go.