The guard looks up in surprise, and gestures. "Sure, go ahead. Not much left to see, just this one spot, won't come out," he says, scrubbing at it anew. There is a slight, lingering aura of magic, but it's very faint, and clearly fading, just a lingering trace. (Know: Arcana, DC 15 reveals hints of the Necromancy school.) Careful smelling reveals only the scent of soapy water and old stone,, though she does, eventually, catch the metallic flavor of blood in the smell, though not strongly enough to identify its origin. It certainly seems to be imbued deep into the stone, and it seems unlikely that any amount of scrubbing will remove it, now, short of carving out the stone.
Unfortunately, before she can get a scraping, the guard, already a bit wierded out by her smelling the stone, asks her, politely, to move along, so he can keep working.