As seen in:
Night of the Living Dead, Diary of the Dead
Why it's a sign:
Don't be fooled by their barely functioning nervous system and shambling gait, Zombies have a devastating sense of timing. And if there's one thing they appreciate more than brains, it's dramatic irony. So if you're walking through a graveyard with a woman who's scared of the undead, you should avoid putting on your most sarcastic spooky voice and saying "They're coming to get you, Barbara." For whatever reason, sarcasm sounds like a dinner bell to zombies.
She's not reacting to your devastating sarcastic/spooky voice.
Zombies are incredibly patient. You can go your whole life avoiding any zombie irony--never dressing up as a zombie on Halloween, skipping all the "dead people are dead and never coming back" parties your friends throw every Easter. No rush. Take as long as you want. Zombies aren't busy. They'll just be chilling out under a thin dusting of suspiciously loose soil. And the second you slip up and do something that would make a zombie attack the least bit ironic, like say, starring in a zombie movie, they will be on your ass with a quickness.