There are three things that can lead to your problem:
–cracked bowl that leaks water;
–faulty waste vent;
–siphonage due to material (cloth/paper/etc.) caught
in the bowl trap.
The first you’ve apparent excluded…the leaking water will soak the floor under the toilet. Make sure that isn’t happening.
The second usually causes the water to be sucked out of the bowl with a gurgling sound in a series of “waves.”
The third usually siphons the water from the bowl more slowly and continuously than would a blocked or missing vent.
The first thing you should do is plunge the toilet, then run it through with a closet auger. If that doesn’t work, snake the drain line from the vent on the roof (you better have one…if you don’t, that’s your problem!). If that doesn’t work, remove the bowl and snake through the closet bend.