My car is a 2004 Mazda 3. This happened with the last battery I had as well as this newer one.
Basically if I leave the car sitting for 2-3 days without driving it, it doesn't have the voltage to start and I need to trickle-charge or boost it.
I recently took a multi-meter to it after driving it then letting it sit for a few hours - 12.1 V. Not fantastic, but good enough to get the job done. I used the ammeter function to test for parasitic draw and found less than 50mA being drawn (if I remember right it was 16mA or possibly 1.6mA.)
So now I'm stumped. Sure, it could be a bad batt, but this happened with the last batt I had too. It doesn't make sense to me that there doesn't appear to be a parasitic draw yet the thing drains after two to three days.