Honestly, that’s the part that really gets me on rereads. Harry manages to come up with “He’s got Padfoot in the place where it’s hidden” and Snape can’t manage to work the word ‘mystery’ in when Umbridge asks him what it means?) It adds to the tragedy of it all that the one time Harry really tried to plan, it all fell apart anyway.
Forget Dumbledore. If either Fred or George had looked at the Marauder’s Map and noticed that their little freshmen brother was spending his nights curled up with some dude named Peter Pettigrew, half the wizarding world would still be alive.