Being called in to investigate a building whether residential or not is a tricky thing. Let’s forget that you actually need to know what you’re doing, but everyone expects everything to be like what they have seen on Paranormal Television.
Let us forget that people seem to stick to investigating in the dark for no apparent reason than to make it scarier. I'm referring to evidence in this case (the latter is a rant on its own- see Ghosts only come out in the Dark). Paranormal Television would have everyone believe that things go bump in the night every time and everywhere, people run into spirits every time the lights go out. This isn't an episode of Supernatural or Constantine- Nothing could be farther from the truth.
Investigating can be like watching paint dry. Sitting around expecting or wishing something to happen just to have it fall short. Actually, this is what will usually happen. Even if there is something residing in the area being investigated, it can take several times being there for even the slightest bit of activity to happen.
If paranormal activity is what you crave, I suggest sticking with paranormal television, with a reminder that it's just that and just entertainment.