Yeah, EMF detectors are usually used to explain why people get creepy feelings in certain places because of things like electrical crap, but they're also supposed to have a like to the paranormal because of the theory that entities use energy to manifest themselves or interact with people or environments. It's supposed to be that you can detect and entity's presence by getting an abnormal EMF reading because of the energy it produces.
And now that it's been brought up, i agree that there's a good possibility of the black box being an EMF detector. What it has to do with Slendy i don't know, because (to me) Slendy is a physical presence and not a spiritual one, so i'm not sure why he would need to use energy. But who knows.