At the former apartment, my office/work area had an opening to the living area high on the wall that might've been for a window, or something. My cat spent a lot of time staring at that opening. Not just the opening itself, but he'd work his way backwards, along some exposed AC ducting (it was a loft with 20' ceilings), trying to plot a route from the floor over the ducting to the hole. If we'd had a high enough shelf in the living area he would have gotten in.