Show me.
It took Hal quite a while to figure out Emily's alter-ego and it had not been entirely his fault or because of his lack of attention to detail, It was all because of Emily/the Masked Lady's mask.
When Hal had encounter Emily outside of Ford's study, Grimoire had said it was unable to read her cultivation base and the reason was most likely due to an Artifact she had with her.
At the time, Hal had considered it a coincidence that he had not been able to sense the Masked Lady's cultivation as well but had thought nothing of it because, for some reason, he had not been able to mark a resemblance between the two.
That said, there was simply something he could not seem to place about Emily and it was what had finally allowed him to make the connection.
When he did, he asked if Grimoire had known all along and the Artifact told him once again that while it had impressive functions, it was far from being clairvoyant.