The note has no signature - only a declaration of someone absconding with Christopher Mango to the therapy graves. And while Fever could ponder about who's maintaining them now that winter's over and lying in a grave would be less of a miserable experience, Mango's disappearance is worthy of investigation. Why tell her specifically? Better her than Degas, or Mulcahy, she supposes - both souls who don't need the stress. But she has to go retrieve him, else Mulcahy will count him missing at a point. 

As she approaches where the graves are, Fever deliberately keeps her footsteps light. Not so much that she'd be sneaking up, but nor is she going to give them too much advance notice that she's coming. 

"Got your note," she calls out, coming nearer and nearer to the graves. "Make your demands for his safe return, though why you want the beast is a mystery."

She hasn't seen who it is yet, but she's approaching rapidly.