She's sure Katniss had told her, but Johanna filters information in and out in the order of which things matter to her. Where Peeta works? Fairly useless, if she's honest, but it's information she tucks away in the event she needs someone in the kitchen. She goes back to her digging, just as he's asking her what she's up to and she knows she probably won't get away with fibbing. At least, not with another victor. "I'm getting my weapon out of retirement, just in case."

Just in case a door comes and she has to fight, just in case she has to go back, just in case she can change the future.