Toilet Water Level Drops

I have noticed that when I flush my Toto everything works as expected. Later when I come back the water in the bowl had dropped down quite a bit. Flush it again and the same thing happens. Throughout all of this the toilet works fine. Any ideas?

If you add water slowly to the bowl with a pitcher does the water level rise?
It may be either the normal water level in the bowl regulated by the height of the trapway, or, there may be a crack in the trapway allowing water to leak down the drain. This crack would be internal where you can't see it.