Tracing cable in wall
The telephone industry has long used a device called a "Toner". This is a two part device the first of which is connected to the cable to generate a traceable tone. The second part is the tracer. Normally it is used to identify a particular pair on a cable but it that can, with fairly good accuracy, be used to trace the location of the wire. 

Inexpensive versions of Toners can be found on the web including one for ~$20 from Walmart.com.