Well, it's true people would come look in the house, if (say) it was me who'd disappeared.

But -- as I understand it, the Fidelius charm makes whatever's under it kind of disappear. And it's not like it's a big empty space; anyone looking won't notice it's not there.

If we put the whole house under the Fidelius charm, people would come looking and find the house missing and say 'oh! it must be under the Fidelius charm!' and that wouldn't do at all. But if we put just the east attic under the Fidelius charm, people wouldn't even notice that part of the house was missing. If that's even possible, which it might not be, I'm not sure.