From: ep. 3.0 (short “Many Happy Returns”)
Object: in the box of Sherlock’s things Lestrade gives to Watson, we see several oddities, including a yellow full-face mask.
Reference: In “The Yellow Face,” the mysterious plot centers around a child who is disguised with a yellow mask, to hide what she really looks like. It’s definitely one of the more Victorian-melodrama-flavored stories in the canon and so Mofftiss are unlikely to use more elements from this story than this brief nod.
Watch “Many Happy Returns” here: Many Happy Returns