Acoustic Locating & Mapping

In a recent conversation with a client, they mentioned that they are regularly called out on an “undetectable” locator ticket.  What is this "undetectable" stuff?  To them  “undetectable” means something like “unable to locate the structure using standard electromagnetic devices”. In many of these cases...

What is acoustic locating? According to Wikipedia: “Acoustic location is the use of sound to determine the distance and direction of its source or reflector. Location can be done actively or passively, and can take place in gases (such as the atmosphere), liquids (such as...