The toilet in the guest bath is seldom used. When I just went in to be sure it was clean, the water level had dropped way down, there is only about 3" of water in the lower area of the bowl. The toilet is on the main floor of a single family house. The other toilet backs up to it in a different room and is used constantly. I don't hear any gurgles. Why does it drain?

Water either evaporates ( slower changes in water level) or it is siphoned out ( rapid water level decrease ).

And a few more: Cracks in the bowl and pets drinking out of them.

And one more! 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. To check the bowl must be pulled, filled and put up on saw horses. You can then see if there's any leakage from the bowl.
Sometimes a blocked vent can "vacuum" water out of a bowl. Does the level "bounce" up and down?