Hee are some ideas:

Shared dryer vents?

Shared Stove top cooking vents?

Shared plumbing stack venting? unlikely but if there is one missing
missing trap on one sink somewhere and the plumbing venting is shared
then air may be able to travel...

Chimney sharing?

Light fixtures on the ceiling? check for air leaks, it's a stretch but
worth a try.

Somebody mentioned electrical outlets

how about attic access doors??? silicone seal the cracks in the attic
door access to stop airflow and if you ever need access back into the
attic, it's easy to cut and reapply new sealant

Something between the garages?

Central vaccuum venting? or collection tank?

Air moving between the floor joists?