A toilet bowl's water level normally stays at the same level until you flush the toilet, quickly returning to its normal level again. Certain problems with the toilet can cause the water level to drop as can your usage habits surrounding the toilet. You may need to check out multiple possibilities to solve the mystery of the disappearing water.
The trap sits under the toilet bowl and curves to hold water inside. On some toilet bowls, you can see the trap from the side. The toilet bowl and base are made of porcelain, which can crack easily. Hard blows, contact from heavy or sharp objects and temperature changes from pouring hot water in the bowl can cause the porcelain to crack. Once the toilet's trap cracks, water can escape. The leaking water will drop the toilet bowl's water level as well.
If you have a pet in your house, the source of the dropping water level in your toilet's bowl may be from the pet stopping by to help itself to a drink. While the thought of your pet drinking from the bowl can seem harmless, that drop in water can drain the toilet's trap partially or completely. With the trap dry, there is nothing standing in the way of sewer gasses down the drain line from coming up into the toilet's bowl and your bathroom, filling the bathroom with toxic gasses that can suddenly combust.
If you do not use the toilet regularly, the water level could be dropping just from evaporation. The humidity level in your house will determine just how quickly the toilet bowl's water level will drop. The drier the air, the faster the evaporation will take place. Flushing the toilet more regularly will keep the bowl's water level normal.
A partial clog of toilet paper in the trap area of the toilet can lead to water siphoning out of the bowl. When the toilet paper sits over the bend in the toilet's trap, leading down the passageway to the drainpipe in the floor, it will slowly draw water out of the toilet's trap and drain the bowl. Using a plunger, you can force the partial clog out of the toilet and down the drainpipe in the floor where it belongs. You must first flush the toilet so the bowl water level returns to normal. You may need to pump the plunger a few dozen times to get rid of the partial clog.