Proximity... a perfect detective.

A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact.
We have capacitive, magnetics, optical, inductive and others.
A typical inductive sensor that you can find in an industry is one like in the following picture:

http://ilabzelectronics.com/Inductive-proximity-sensors

It is a type of non-contact electronic proximity sensor that is used to detect the position of metal objects, you can find in labelling machine for example, valve position control during processing , position detection, etc.