I had a very similar situation years ago. Could hear something scampering along the ceiling joists, just every 2 or 3 days, usually in early evenings. Snap-traps caught nothing, so I put in a large live trap, again with peanut butter. That night there was a loud commotion up there, and I went up to find a very large squirrel bouncing off the walls of the trap, frantically trying to get out. Turns out he managed to squeeze through a softball-sized opening between the garage and house attic that I never gave much thought to. And he got into the garage attic through the scuttle access that I usually left open, for moving things in and out of the storage area up there--he managed to run up a nearby tall storage shelf unit and then jump (3 ft.+/-) across a bare wall to get in.