The water in my toilet bowl all seeps out with in just a few minutes. What causes this?

The water in my toilet bowl all seeps out with in just a few minutes. What causes this?
There are several things that could cause this to happen.
1) If something were dropped in the bowl and cracked it.**2) If a vent were blocked and it vacuumed the water out. Do you hear or see any gurgles or bubbles?**
3) There could also be a factory defect in the bowl. Sometimes in casting the bowl there is a small bubble in the china. Instead of junking the bowl the manufacture will put a porcelain patch over the hole and send it on through. On occasion the patch will fail allowing the water to seep out of the bowl. If that's the case you would have a full bowl after you flushed and the bowl filled but some time later you would notice a drop in the bowl level. The only other thing might be if you dropped something in the bowl and cracked it. If you're worried about where the water's going, in both cases it drains back down into the sewer and not under the bowl on the floor. Hope this helps . Tom