Some problems you may be encountering.
[br]
SOMETHING is causing a vacuum(plugged vent) and allowing the water to be sucked out of the toilet trap when another drain fixture is used.
[br]
One way you can check for a plugged vent is to flush the toilet, when it fills and the tank shuts off turn on hot and cold water in the closest fixture to the toilet(sink, tub), then watch the water level in the toilet to see if it moves. If this is the problem it should happen quickly(within 2 or 3 min).[br]
[br]
Check the vent where it sticks up through the roof and look for obstructions(birds nest ect..)Look into the vent with a flashlight to see if obstruction is further down.
[br]
If so you can use a water hose to try and flush the pipe.You can also use a plumbers snake to try and pull any SOLID objects back out(rubber ball etc...)
[br]
Problem could also be that there is a crack in the trap of the toilet its self and the water is slowly running down the drain (lucky you ).In this case the toilet will have to be replaced due to the fact that the trap seal is being broken and allowing any sewer gases to enter the house.
[br]
Also if there is any crack in the toilet it presents a dangerous situation to anyone who uses it.[br]
These are not the only causes but are things you can check for yourself.
[br]
Good luck and post back!