EMF meters in vigils; what causes readings
Other field disturbances
Although electrical equipment is easily the most likely source of sudden EMF meter readings, there are others.
Though they're designed to detect fields at 50 Hz, EMF detectors can be surprisingly sensitive to simple movements of magnetic objects. Walk past one with something magnetic in your pocket and you might get a reading.
You can even get a reaction by waving a tin can next to an EMF meter. Even though the can is not magnetic, it contains steel. Steel is highly magnetically permeable. That means it distorts the local geomagnetic field. Moving it disturbs the local geomagnetic field, causing a reading on the EMF meter which sees it as a low-frequency field.
So anything made of steel or iron which is vibrated (like the drum of a washing machine) could cause disturbances big enough for an EMF meter to detect. Even a steel filing cabinet being vibrated by heavy traffic might be enough to get a reading.
Anomalous EMF readings
Paranormal researchers often get excited on ghost vigils when there is a sudden high reading on an EMF meter. This is thought by some to indicate paranormal activity, maybe even the unseen presence of a ghost.
Unfortunately, there are a great many possible mundane causes for a sudden jump in magnetic field readings. Sadly, EMF meters are not good at telling them apart. Since the meter cannot tell you where to look, you'll need to poke around the area and see if you can see a possible source of the field disturbance. If you have a spare EMF meter available, you could try putting it close to the suspect area to try to localise the source.
So what could have caused such a disturbance?
The most likely cause of a magnetic field disturbance in any dwelling is operation of electrical equipment. Such equipment can deliver changes to the local field by being turned on or off or changing state in some way (eg. a washing machine moving between cycles). Electrical equipment is a potent source of magnetic field disturbances.
Some EMF meters can detect electrical wiring behind walls. However, such wires are relatively poor producers of magnetic fields. The high density of wiring and electrical devices in an appliance makes them much better sources of fields.
Not all electrical equipment that could be the source of fields will necessarily be obvious in a room. It could be behind walls or, more likely, under floorboards. Any equipment that contains relays, transformers, capacitors or switches could produce some brief, powerful magnetic disturbances.
Some electrical equipment is on all the time, or most of it, and operates automatically, even in the middle of the night. You should also consider any equipment you've brought with you on the vigil as a possible source.
See, also, this study for other magnetic anomalies.
Why use EMF meters?
Given that all you can tell from an EMF meter is that there was a disturbance to the local magnetic field, of unknown magnitude and frequency, are they any use in paranormal research?
You could use the same argument about most bits of equipment used in vigils. Since no one knows what causes paranormal phenomena, or what effects they may have on the environment, it is legitimate to try measuring any environmental parameter you can to find out. So you can certainly justify using EMF meters on that basis.
The problem is that EMF meters were designed to monitor electromagnetic pollution. The designers were not too concerned about field frequency or direction because that wasn't the point. They only wanted an overall figure for personal exposure to electromagnetic fields.
This means that EMF meters are not really suitable for looking for EIFs (experience-inducing fields) which cause certain people to hallucinate. See the MADS page for more on this.
EMF meters could, in very broad terms, be useful in differentiating between hot-spots (where phenomena have been reported) and control areas (where they haven't).
So what happens if we find 'anomalous' readings but can find no obvious cause? Are they paranormal? On their own anomalous readings are just that. However, if such readings coincided with a report of a paranormal incident, they could be interesting. Even then, you would need to make sure there wasn't a mundane cause for the unusual reading.