That's a good question. Certainly in the Lord Protector's realm, it's easy to imagine all kinds of things one might wish to keep secret -- perhaps Megan's guardian herself once had a relationship with a Muggle or a Muggleborn. But maybe it's just that she's a sad old lady, who, despite her loneliness, won't let her half-blood ward close enough to be a real part of her life.

Even if there is no larger, plotty significance to Megan's prying in her guardian's attic, it was a great bit of story-telling for showing us Megan's character and how she grew to be so careful and self-censoring.