Whenever I get to a certain area/place, an unknown icon appears at the status bar next to network signal icon of my K800i phone and then disappears as I get further away from the place.

I have drawn it on MSPaint for you to take a good picture of what I'm talking about. (Well, it's not an accurate copy of the icon but I tried my best to make it appear very similar.)

Anybody knows what it means? Here's the image: