My boss has asked me why the level of water varys in his toilet even though he hasn't flushed it?

my boss has asked me why the level of water varys in his toilet even though he hasn't flushed it?
There are a few reasons the bowl level would fluctuate. Wind blowing past a vent could cause a vacume that sucked some of the water out of the trap in the bowl. There could be a small crack that allows water to seep out over time. There could be a bad flapper that allows water to leak out of the tank and fills the bowl. Take your pick. Regards, tom