That's a level of unnecessary pseudoscience. Live people have a smell even if they aren't infected. It isn't like we need to have the zombies attracted to infected people, but not attracted to uninfected people - they just come after people. So, they just need to smell or hear or see humans, period.
And the implication is that they don't just come after humans - any mammal, at least, seems to be okay food to them.
Except generally, they don't. They usually attempt to beat down doors, rather than use doorknobs. I think we can chalk it up to writing in early episodes that later just got dropped.
On the smell thing, in the show, zombies have been fooled by zombie goo being coated on people or even zombies around a person, so they are not sensing life.
Rumor has it that this season, we see the "smarter" zombies.