"ENTITIES FROM BEYOND?"
EMF just means electromagnetic field which is caused by raised ammounts of energy either in the presence of a paranormal entity, or...just simply detected around homes and facilities due to the ammount of energy running through the place. Electric energy even. We all know entities like to manifest through energy...so...they will drain any batteries, or energy available from those electromagnetic fields in order to manifest.
K II meters are great detectors of waves that are non visible or audible to our eyes along with electromagnetic fields. They have blinking lights that can even be used to communicate with entities if there are any present. Those are awesome to identify the presence of energies like that. Because they will light up according to how strong the energy gets, etc...
You can detect non physical things when they make use of things existing in our world such as energies, smells, etc...if you cannot really find an explanation to sources of why those things are increasing or happening, then you know it is a paranormal thing which does not necessarily mean it is a "ghost" but just paranormal which simply in other words is "abnormal OR abnormal BS" occurance.