Freek, IMHO if you get a compass (the thingie that scouts use) it will be a better ghost finder than the cell phone app. As we know the compass will point to magnetic North. If it points in another direction it could be because of 2 reasons. 1) a strong magnetic pull from something normal (example: metal near by) or 2)
maybe ghostly activity.
Since
we think ghosts are made up from electrical energy or draw energy from the surrounding area to manifest themselves (even ghost hunters will disagree on these points) a change in magnetic field would make the compass react. If the compass needle spins around for no apparent reason there
may be some paranormal activity going on.
I've seen seasoned investigators go crazy because their Kll meter was going off. Then they found out someone set their cell phone to silent mode when it should have been turned off. The cell phone contacting the tower checking for messages set off the Kll.
Be careful if it uses a 'quantum flux capacitor". Then it becomes a time machine.
(Back To The Future)