I think ventilation systems are definite possibility. You might remember the Legionnaires' outbreak in Barrow about 20 years ago, where a number of people died. That was a complete puzzle regarding transmission, until it was found that a faulty AC system in an arts centre was venting contaminated air into a narrow alleyway in the town centre. That explained the random pattern of infections.
We have had quite a few people who have caught SARS-CoV-2 when in hotel quarantine. So aerosol transmission does occur.