[ol]
[li]Fill the toilet tank. Measure the water depth.[/li][li]Now go away.[/li][li]When you come back later and find the bowl empty, but the tank is still full, the problem isn’t with a leaking flapper.[/li][/ol]
Not quite the same thing but bleach in the toilet bowl can make the level drop. I think it was bleach anyway that mum used to put in there!