They discover it by accident. They're still new to their territory; naively so. Greyfern volunteers to test the herbs, because Brightstar promises to give her someone good. Not Voletail, with his leery smile, or Brownclaw, who is literally the blandest cat she's ever met. So she eats the herbs they find, and it's mostly alright. She feels ill a few times, but they mostly stumble across herbs they knew in the old forest. But then the berries are brought back to camp. Greyfern expects them to be sweet; instead they're sharp, and she'd rather spit them back out than swallow.
Minutes later, Greyfern starts to burn. There's a fire within, seething to get out, and the silver tabby realizes, as she writhes, that she won't get anyone at all. The inferno spares nothing.