The second young miss didn't want to let her bodyguard out of her sight or hold, so she needed someone to go on a grocery run for them. Habitually, people dumped the task on Mari. In fact, lately, anything regarding the second young miss was being dumped on her.
She always returned looking dejected and exhausted, sighing deeply in the corner. The more miserable she looked, the more the others pushed that kind of stuff to her.
That was how Mari easily visited the annex without any suspicion. Her life was already miserable before, so acting it out wasn't hard.
Of course, the grocery run was real. Rather than a chore, however, it was a fun time since Mari could get out and be away from the other workers. Not to mention, the young miss told her to get some snacks and allowed her to choose a few for herself.