Well, all snarkiness aside, Martea is described as having these "slits" where the people she devoured are preserved within her Abyss Worm body. I think, if you want to put the sword on her, erm, person (for lack of a better term), an additional slit for the sword or one the sword has to share with a victim is as good a hiding place as any
. I don't think, she'd actually use
the sword like a weapon, the Abyss Worm attacks are better than anything she could do with a sword in that form.
I'm brainstorming as I'm writing this, so please just bear with me, if this is becoming a little incoherent... I'm just thinking, when the group actually attacks Martea and one of them is devoured - maybe they could become aware of the sword inside her body that way? That might actually be cooler than the abandoned caves idea... I'll always have that as a fallback, I guess