My toilet bowl empties itself from time to time and I then get sewer gas smell. There is no particular pattern as to when this happens. And, when it does, the tank is always still full. I live on the third floor of an apartment building, and my downstairs neighbors have never experienced this. Any ideas?

Vent stack is blocked or partially blocked somewhere above your toilet drain... and water is getting sucked out of yours as it's the point of least resistance.

Talk to your building Manager... this is their problem to resolve. I'll bet someone else has experienced this as well in your building.

The Smooth one nailed it! You have a blocked vent and when someone drains or flushed it puts a suction on your toilet drain line. I' puzzled that you don't hear and gurgling from the traps. The solution would be to snake and clear the vent, Good luck, Tom