In all actuality, Marietta didn't, either (although some part of her felt like she should), so she was glad to let the subject drop.
"I...er, well, I was going to the Library and then suddenly I just forgot where I was, and then I wandered around some more in hopes that it would remind me and I got even more lost." That probably sounded like a crazy excuse to someone who didn't know what had happened to Marietta's mind and memories, but she was never very good at lying.