The house has 3 ranks of the Concealed feature, which adds +20 to the DC of any checks made to find the house or learn anything about it - Gather Info, Knowledge, Search, etc. The descriptor is the psychic perception filter created by the house's unholy aura. All 3 heroes have skills high enough to find it eventually, but none of them would be able to just take 10, which is why I described it as taking multiple attempts to keep it in their minds.
EDIT: I don't need any actual rolls from any of you.