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A “web bug” is a tracking device consisting of an object embedded in a web page or e-mail, that unobtrusively (usually invisibly) reveals whether and how a user has accessed the content. Other names for a web bug are web beacon, pixel tracker and page tag. more
Want to check who is secretly bugging you? Little Snitch for OSX does an excellent job and offers a free trial. Similar products exist for PC based computers. ~Kevin