“The concept of 'evidence' had something of a different meaning, when you were dealing with someone who had declared themselves to play the game at 'one level higher than you'.”

--Chapter 86

ETA I was referring to my pet suspect for the hidden hand (who's clever enough to play Dumbledore and Quirrell against each other and leaves scant evidence of her existence). But I think Eliezer was referring to Defense Professor having a high prior and being Voldemort:

“The Potions Master said dryly, "The Defense Professor is always a suspect, Mr. Potter. You will notice a trend, given time.”

--Chapter 79