Actually seeing these things is a form of measurement.
If what's being seen is from some weird kind of quantum entanglement between so called potentially parallel universes (which as I understand the main related theory means it's not so much "parallel" as dimensions warped up inside the space in between our own on the Plank scale) ...then it makes sense that the phenomena isn't fully recordable via normal means.
Furthermore: If it's been happening through out human history...then one can conclude that:
A) It is just what people seem to think it is IE the supernatural etc (which also means our understanding of physics is still a lil off since it needs to include the study of such things in more than just a fringe way..
B) It's a naturally occurring phenomena of the physical world and we humans are categorically misinterpreting what we are seeing or hearing etc and we just haven't figured out a way to substantially "measure" events....(or the act of attempted measurement is changing the results just like it does in the famous split screen experiments).
C) It's a relatively uncommon yet natural occurring element of human experience to "see ghosts" or at least "think one has" on occasion.
D) Some combination of the above with or without adding other unknown factors or postulations.