Either the smell was wearing off (unlikely), or the walker had been standing next to her long enough by the fence that he finally caught a whiff of ‘live person’. And then once the first one noticed, the others swarmed.
I think they smelled fresh blood from her wound.
Yeah, they showed her putting her hand on her wound, then reached up to grab the fence. Her hand was about head level, and the walker turned at this point as if he smelled something interesting.
Clearly this is why bears and zombies attack women who are menstruating.
Here is a spoiler about the finale from Australian TV.
Link.
Kind of a nitpick now, but an older (pre-digital) PBX phone system in the prison would likely have its own local batteries. If so, they would have been recharging when Andrew was running the generators, so calls within the prison would work. A digital PBX would also have batteries, but recovery after the batteries run dry usually requires a tech to boot the system.
We just started this series a few weeks ago, and now we’re just one episode behind the rest of you. We’ll be caught up when the season resumes next week, so I’ll be able to read these in real time. Can’t hardly wait!
But now you’re screwed, for you must wait a week for each one! 
I heard the theme music when I walked into the pharmacy. My grumpy pharmacist was turned onto the show by one of those students who works for him.
He’s run out of DVDs and is more grumpy than ever.
Caught up now. Even though we’ll have to wait a week at a time, now I get to speculate with all of you, plus co workers every Monday morning!