Apart from EVP, another area I am particularly interested in for explaining apparitions - or whatever terminology one chooses to use - is the concept of parallel worlds; that everything that ever has happened, is happening or ever will happen is actually all happening at the same time. That we are not actually seeing ghosts, but the real thing; they are bleeding through to us from their own time warp.
This, for me, would explain the mass apparitions and noises associated with ancient battlefields and even small 'residual' phenomena. I accept that this may not be a satisfactory explanation for intelligent or demonic hauntings.
Having said that, I recollect an episode on Taps when the team were doing EVP work in a hotel bedroom, and they asked "Can you see us?" and a voice came back "Where are you?"