As I embarked on my summer travels, my friend, @stephanwhite, kept emailing me disclosing my latest known locations. He was teasing me that he always knew in which city I was. So I first thought he was simply following my foursquare userpage or umapper trail; I was wrong.
What he was really doing was going to the following page: http://foursquare.com/search?q=f1vlad&x=0&y=0. Then he was clicking tab "People (1)". That was it. 
I can hardly call it a hidden feature. But neither is it an obvious one.
From there, I created a simple python script that could always show my last known position. Feel free to use it: http://github.com/f1vlad/4sq. I made it run every fifteen minutes via linux cronjob. There are more setup details in README.