Using iStat (which provides a small graph in the menubar showing the present upload and download speeds), I notice once in a while that something is being downloaded, without any action on my part. Since I allow most of the programs to download all the updates, any of them, plus MacOSX updates, plus Dropbox etc. may be the culprit at any given time. But is there a quick way to understand what exactly is being downloaded at a given moment (for instance, to stop it if the case might be)?
There are different ways of doing this. The easiest way is to use an application designed for this purpose like Little Snitch which has the option of showing a window that displays what files are being downloaded/uploaded and to where.