In a mobile marketing context, we use 'proximity' most often to refer to the consumer's mobile device being within range of a beacon/hotspot/antenna of a proximity marketing system. The mobile device is simply present, with the appropriate connectivity options turned on, and the discovery is left to the system.

However, a different take on the term refers to using the mobile device itself as the proximity marketing sensor. The mobile device takes the discovery role, meaning we can use it to find things near us, see if something is close to us, etc.

I came across this YouTube video just the other day that shows how this concept would work:

DecaWave ScenSor in Action
Interesting ideas and concepts in the video and though I'm not too sure how much of what is shown are off-the-shelf products, this is definitely something I can see us using a couple of years down the line, especially given the great push location-enabled and location-based services have recently got, after a number of years of always being on the verge of happening.