I had another though or two.
the chase/clearance for the flue pipe up out roof could be air flow route.
the floor joists at each level could have openings to the exterior of home (band joists run around perimeter and floor joists butt into them). its likely plywood joints hit right at thsoe locations as well, giving lined up gaps for air intrusion. I'm assuming there is plywood on 100% of exterior as sheathing
bathtub plumbing openings in floors, the stud cavities around the tub, an if a tub unit w/ integral top or has dropped ceiling over tub, plenty pathways there from floor to floor or into attic if on top floor.
pvc radon riser, other plumbing, duct risers and the gaps around where penetrate floors
voids inside walls where framed out for soffets, angled walls, etc.
all these could be interconnected paths for airflow inside the walls where you can't see all them.
Had one customer, return was in closet floor pulling through louver in door. the tub access door was gone, had been pulling air from attic through drop ceiling above tub, into wall studs and down through space under tub into return ductwork. sealed it up and his tub doesn't get cold in winter, hot in summer.
you might try looking for "builders guide" book for your climate, Joseph Lstibureck
Col 3:23
questions asked, answers received, ignorance abated