Q: What if a haunting is not around while someone investigating? Does it mean its not haunted?

A: Depends.... There are reasons at times that people can claim a house to be haunted
While most of the claims are genuine there are some who would exaggerate to boost tourism to a facility or to get a higher price for a house (yep...haunted houses are back in fashion)
Also we may come across a practical joker or two or they simply just mistaken.
However keep in mind that some hauntings are particular to certain phases of the moon, only happen at night, at certain dates or anniversaries of events or in the case of some ghostly "recordings" the energy simply fades away. Some people believe that dates such us Friday the 13th or blue moons would make a good date with a ghost or two, waiting for them to surely appear.
We have to separate some myths from folk knowledge and parapsychology, in this case Friday the 13th is more important to us than to spirits or ghostly events due to movies or superstition but it has no significant influence on energies. It is interesting however that Halloween (Samhain in pagan practice, All Hallows' Eve in Catholic festivals or the Day of the Dead in certain traditions) was chosen to be 31st October/1st November as they have observed unusually strong spirit and ghost activity on these days.
It is better to follow a Moon calendar to chose the right date. Spring and Autumn Equinox, Winter and Summer Solstice are special dates because of their influence on our surroundings , animals and of course energies.
There are also ways of "recharging" a haunting, be it by accident or purposely, although we strongly advice against the later. The laws of magic states that similar energies attract each other and since ghosts and spirits are energies it is possible that a certain smell, similar event, music, a particular person's presence that has similar energies to those departed, an item belonging to them being displayed or present, them being remembered or mentioned regularly or simply going through the same emotions as they did can feed a haunting with new energies. There is also something interesting about a place being renovated where the haunting used to be that can cause an increased activity. In the case of elementals and certain spirits disturbing of the landscape can have consequences. Such beliefs are found in the traditions of the Incas, Mayas, Aztecs, Australian Aboriginal culture etc.
Of course this works the other way around and sometimes there is something about the people visiting the place than can cause ghosts and spirits to be avoiding the place for the occasion.
It is useful to keep a diary of events recording everything you feel, see or hear and eyewitnesses are always handy although we understand that in cases they are hard to find.