Harry told his patronus to specifically seek out Hermione, which lead him to the Troll. He also got a response back of "AHHHHHHHHHH", which seems suspicious enough, but not completely solid evidence. These details don't seem to match up with the idea that somebody else died in Hermione's place.

I think we are meant to assume that Hermione did die, or at least experienced something close enough to death that Dumbledore was alerted.