Anyone able to confirm my theory or come up with an alternate theory for the operation modes?
Definitions:
standard filter -- the smaller air filter (can't recall, but Elon may have referred to it as the secondary filter); I refer to this as the "standard", because it is likely the standard type of filter used in most all cars.
HEPA filter -- the big air filter that does the fine particulate filtering
My theory was:
1) Standard -- Air from outside is passed through standard filter only.
2) Recirculate -- Air from inside is passed through standard filter only.
3) Biohazard -- Air from outside is passed through standard and HEPA filters.
Reasoning:
There's been scuttlebutt about Biohazard mode creating a positive pressure environment inside the car. That's not possible unless you use outside air (or some air reserve in a tank or something, which is highly unlikely, so I am dismissing that possibility). But if the Biohazard mode is using outside air, what makes it different from the Standard mode that uses outside air? It could blow air more forcefully, but I would speculate that the Standard mode doesn't use the HEPA filter at all, and that the Recirculate mode doesn't either. After all, most of the time you don't need the advanced filtration, and this would likely greatly extend the life of what is probably a very expensive filter. It may also be noisier when the HEPA filter is in use which would be good to avoid when not needed, thought maybe not an issue given how large they made it.
Note that regardless of outside versus inside air with the Biohazard mode, either way it would help with interior gas releases (from you farters out there; you know who you are!). For inside air, it would be treated by the filter, and for outside air it would be disbursed by the positive pressure environment with interior air being replaced by exterior air.