Whitey, what you are missing is that people put out moisture too. This is not something I have given a lot of thought to, everyone who lives in my area, Western NY knows that if you use recirculate in the winter, you can’t keep you windows from fogging up. You are a;so tracking snow ,slush and ice in every time you get in the car and it builds up on the floor and freezes every night and releases a lot of moisture into the air as the heater warms up.
Having grown up in Western New York (Buffalo to be exact), I was blessed with a quality elementary education, and having read the OP’s comments, I realize this OP only runs the air conditioning in warm weather, not in cold weather. 
On a serious note, if you’re running your A/C in cold weather, I hope you’re doing it with the heater on, which would totally fall in line with my belief that this is something you only need to concern yourself with when you’re running the heat, and it makes perfect sense not to recirculate the air, because, after all, you’re running the heat and the A/C at the same time, which the OP isn’t doing.
Every air conditioned car I have ever had runs the A/C when you have the defroster on, which is pretty much all Winter and Spring around here.
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Most of my customer’s cars that I work on have foul smelling A/C systems, they keep the A/C on recirculate and make no effort to allow fresh air into the car. When I start the car I lower both front windows and switch the A/C to fresh air while holding my breath.
Respiratory moisture contains bacteria that collects on the evaporator coil, this “cooks” in the hot HVAC housing while parked, producing the foul smell when the car is started.
Dirty A/C systems are common here, plugged condensation tubes are not common in my area, perhaps that is a problem in areas with insert problems.
I saw an earlier comment mention mice. What you are describing sounds just like the smells I got before I found a long-dead mouse in the ventilation system. Mice are a real problem on Toyotas in general and the Prius, in particular. LIke you, I park in a garage and have never seen mice in my garage, but somehow one certainly crawled in and made itself at home in my ventilation system! (If you do an Internet search for “mouse in car ventilation system,” the majority of all results will concern Toyotas. There must be something in the way they design the ventilation system that mice just love.)
If you haven’t done so, I suggest the following home check/repair:
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Remove the lower glove compartment. (It just snaps out by pressing in on plastic pins; no tools required.)
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Reach behind the glove compartment and pull out the air filter.
If you have a mouse (living or dead), you’ll almost certainly see the evidence on the air filter. Evidence will include not only mouse droppings and fur but also bits of chewed up insulation. (Mice chew it up to make nests.)
All of the instructions for removing the glove compartment and checking the air filter are in the owner’s manual.
Good luck!