Have you bought a new toilet yet? I ask only because of how old your post is. I just joined today, was looking for any posts with my shower problem, and yours came up in the search results.
I had your EXACT problem. You toilet has a crack in it, and is leaking down. Your toilet is exactly like your sink in terms of flow path. You have the large bowl, then the double U bend beneath it to trap water, thereby sealing off sewer gasses from coming into your home, or in the case of the toilet, to hold water in the bowl. When you flush, water from the tank enters the bowl. The weight of this additional water pushes the water already in the bowl out the U bend and down the pipe, using the siphon principle to empty the bowl. If you have no water around the toilet, then you crack is in the bend between where it turns up, and then back down to the floor, allowing the bowl to empty into the down pipe.
Anything can cause this crack. Mine looked like a casting flaw, but I'm told just age will do it too, as these are just big pieces of porcelain, and are fragile as such.
Hope this helps,