This RF Wave Absorbion meter will detect signals between 10cms and 10m away all acording to the setting of the 22MegOhm gain adjustment potentialometer and the power of the transmitter.
http://geocities.com/ajpotts19/wavemeter.html
detects very small currents such as those caused by leakage and changing electric fields
http://www.4qdtec.com/putpr.html#efld
handy when you have to follow the mains wires buried in the wall or even water pipes provided they are not too far away (2-4cm max)
http://www.geocities.com/CapeCanaveral/Lab/5185/circuit.html#ELECTRIC
The system can detect human and animal motion by the electric fields they disturb.
http://www.imagineeringezine.com/e-zine/efield.html
uses radial type inductor as probe and responds well to low frequency changing magnetic and electric fields. headphones are used to for detection
http://www.mitedu.freeserve.co.uk/Circuits/Misc/emf.htm
http://cs.okanagan.bc.ca/ve7ouc/eng/kc6wdk-mirror/fsm.html
http://www.edn.com/archives/1996/101096/21df_07.htm
http://www.geocities.com/tomzi.geo/fsm/fsm.htm
Schematic of North Country Radio RF Field Strength Kit
http://www.northcountryradio.com/Schematics/rffsm.gif