A toilet that slowly looses all of the water out of the tank or bowl when it has not been used for a period of time is experiencing a condition that renders the toilet almost unusable. When you flush a toilet, the water in both the tank and the bowl empty out, only to be immediately replenished with fresh water.
A clog in the toilet's trap can lead to the water slowly draining out of the bowl over a period of time. The trap is the curved portion of the toilet's drainpipe, located just under the bowl. You will not necessarily notice the clog in the trap, though, because it is only a partial clog, meaning the toilet still flushes. The toilet paper sitting in the trap will pull the water through the trap and down the vertical drainpipe, eventually leaving the bowl empty. Plunging the toilet's drain or using a drain snake will eliminate the partial clog and keep the bowl full of water.
Vent Pipe Obstruction
A toilet connects to a vent pipe, like other plumbing devices in your house. The vent pipe helps replace the air that is flushed down the drainpipe along with the water. A vent pipe's clog may come from a bird that has built a nest on the pipe's opening in the roof, a ball that has become lodged in the pipe or leaves that have fallen into the pipe's opening. Aside from obstructions, the vent pipe may be damaged or broken behind one of the walls in the house. With an obstruction or break in the pipe, the lack of air in the plumbing will drop the water level in the toilet.
A toilet bowl's porcelain will crack easily if you deliver a heavy blow to the bowl or change the bowl's temperature by pouring hot water inside. Over-tightening the nuts on the toilet's base can also crack the porcelain, which you can avoid by tightening the nuts only until you feel resistance. Cracks of any size on or near the toilet's trap can allow water to escape the trap, draining the water from the bowl. Signs of a leaking bowl include water around the toilet, the floor bowing or sagging around the toilet and water collecting on the ceiling directly below.
The toilet's tank can also empty itself out without flushing, leaving the toilet useless until you refill the tank with water. A worn out flapper valve or bolts and washers in the bottom of the tank will leak water, but the toilet's float valve will drop to the point the fill valve will refill the tank with water, keeping the water from completely draining out. The combination of a leaking flapper valve or bolts and washers in the tank, as well as a clogged fill valve, would cause the tank to empty and not refill. To unclog the toilet's fill valve, you must shut off the water to the toilet and remove the fill valve's cap. Turning the water back on for a few moments will allow the water to flow through the fill valve and empty out any debris that was clogging it before.