Yerars ago they would say that you have a syphoned action wher a piece of material is in bowl and syphonning out the water. Well here is what you have if your vent is stopped up it could syphone the trap when you flush toilet, but what you probably have is an internal crck which you can not see and is losing water, Take tank off bowl if you want and then take up bowl and set it on 2 blocks and fill bowl with water and see if it drips out if so then there is a crck in there and you have to place bowl. Is your Plumbing vented to code. Does bowl flush ok. Do you notice any water on floor by bowl. Let me know what happens and if you can give me more info. might help better.