Make sure there are no obstructions in your vent stack, dead squirrel,
blob of leaves etc.
Other than physical damage to the bowl the only thing removing water
besides a flush would be a vent problem or dogs drinking.
If the vent is clogged then a suction can be applied to the bowl by
other parts of the plumbing/sewer system.
Sometimes in very high winds you can see the water in the bowl move as
the wind passing over the stack changes pressure inside.