Hermione wasn't surprised. Sirius had seemed a bit...stuck when she'd first met him at Grimmauld. Like he was still in his twenties. She made a note to herself to write to him. Maybe she could get further information on Bellatrix. As she understood from her last conversation with Dumbledore, few people were in the know. Harry, Neville and her were likely meant to be the only ones, though Harry would have demanded to tell Ron. She doubted Sirius knew otherwise. He'd probably be at Hogwarts under some delusion of protecting Harry. 