if the wind blows when it rains, it could suck the water out of the trap
Stoppage and disintegrated wax seal of course
It doesn't drain, it evaporates!
The water in the lower toilet is probably being siphoned out of the tank by the negative pressure caused by flushing the upper toilet. This CAN happen if the water level in the tank of the lower toilet is over the top of the ballcock. If this is the case, the tank water is being siphoned into your drinking water! Replace the ballcock with a Fluidmaster right away. If you have a small white fill valve that sits in the bottom of the tank, get rid of it. These are illegal and WILL siphon under these conditions. If the lower toilet runs AFTER the fill cycle of the upper toilet, it may be that the ballcock seals at a lower pressure, and the pressure spike when the upper toilet shuts off causes it to run a bit until the added water closes the valve a little tighter.
Usually caused by drop in toilet bowl cleaners. They also eat away the rubber flappers
Probably the drain. Dry it off with paper towels and then run some water through it. Start at the trap and use a piece of toilet paper that will show the slightest trace of moisture. If you find it at the trap, work your way up at each connection until it remains dry. That will tell you where the leak is. Could be at the bottom of the sink. Added:: You should also check the water supply lines that run up to the sink in the same manor as above using a piece of toilet paper...Because a water line can still be leaking without actually dripping water especially in vertical lines,you could also have water seeping from around a fitting and slowly running down the pipe to the bottom of the cabinet
Broken sewer line /stoppage
When Installing The Toilet Back, Get A Straw Cut It In Half. Put One Over Each Of The Bolts Then Guide The Straws Into The Holes For The Bolts. This Works Great And Saves Time And Trouble Getting It Back On. Where the toilet meets the floor, there is a wax (or similar material) ring which creates a seal. If this is leaking, you will want to replace it. Turn the water off. Flush twice to empty the tank. Disconnect the water line from the toilet. (Have some towels handy!) Unbolt the toilet at the base. Carefully remove the toilet from the floor. Remove the old wax seal and replace it with new, available from any hardware store. Replace the toilet, water line, bolts etc. Turn the water on and give her a test flush. You should be all set!
Attach a garden hose to the drain spigot and run the hose outside or into a toilet or sink. Open the spigot and drain the tank until the water runs clear. You could have turned off the water heater beforehand so as to reduce the risk of scalding yourself. You should drain the tank as recommended by the manufacturer. If not, lime scale forms and insulates the bottom of the tank and causes the water heater to use more energy because it cannot heat the water efficiently.
Pressure differences are causing the stop to bounce. This causes temporary water flow in the tank. A new float should take care of this issue.
you are probably pumping run-off water from your well or cistern
Drain may be clogged because of which the water goes dry or if drain pipes are filled with water at the base it pushes air bubbles through toilet bowl water drains slowly out of pipe it also causes toilet bowl to go dry.Hire general plumbing services for help like A-General plumbing services who offers all plumbing repair services.
Toilet
The discharge line (soil ) may be slightly back pitched so it takes longer to drain
If you really mean it flushes but the water does not run away, then the septic tank is plugged.