The Map (spoilers)
Huge spoilers ahead so I am assuming you have seen the movie already.
So, Simon dies and his corpse is left unfound for months on end in the basement that served to hide Tomas. So far, so good. My problem is, when they are conducting the séance you can clearly see one of the psychic team guys tracking the medium's progress using a map.
If they had a map of the house, shouldn't it also mention the presence of a very big basement? Unless they made the map recently and as such had no knowledge of the basement existed at all...but it is a bit unlikely.