Devine intervention, or flying sperm ,,, Females have seeds from a male. It comes from the wind, from some moron letting males grow. I once got them from a guy living 300 feet away. Even though I had triple filters with a hepa filter on the incomming air. That was with a pessurized room ,,The vent fan was on the incomming air add asperated via the vent with a flapper valve on it.
You can pick it up on your chlose , from a bag you bought, Ny NY does a study every year on the pot production of the city. With air samples on some buildings.