I heard that early Christians would make a curve on the ground with their foot and the other Christian would make another curve, thus forming a fish image. Then they could relax and talk Christian stuff.
You heard correct. At one point in time, Christians were persecuted. This is how they made sure they could say "yeah I'm Christian" to a supposed Christian, and not be killed for it.
I don't think any current ones do... at least in America.
I think Bob that there could well be some validity in what you have stated for we know that during the early centuries there was great persecution of the young church and perhaps at times Christians may have used some form of symbolic gesture to identify one another, and certainly the fish symbol was employed.