Building work and ghosts
When people live for a while in a building, they get used to all its idiosyncrasies. They know about the creaking floorboards (that do an impression of someone walking by), the weird shadow cast on a wall by passing cars if the curtains are left open at night, the door that draughts blow open, the reflections from windows on dark evenings, etc. They are used to these things and know what they are.
On the other hand, when someone visits a building for the first time all these things are new. If they have been told that the place is haunted they are quite likely to interpret things that the owners take for granted as paranormal. This is the ‘new house effect'. You should look for it when doing a vigil at a place you've never visited before. It would be useful to have a regular inhabitant of the house with you to explain things and stop you jumping to the wrong conclusion.
An interesting variant on the new house effect concerns building work. It is often said that building work can start hauntings in previously unaffected premises. Such building work may alter the structure of a house and introduce new idiosyncratic noises and other effects. Of course, even the owners will be unfamiliar with such new effects. The result is that they may, sometimes, feel they have an unwelcome ‘guest' in their house.
The new house effect is not an explanation for all ghosts, it is simply the reason for some spurious reports. It is why it is important to try to do more than one vigil at a particular location. It explains why first vigils at a site frequently produce more (and sometimes more dramatic) reports than follow-up visits.