after flushing normally, the water in our upstairs toilet bowl
gradually goes down over the course of a few hours. Overnight it has
gone down about 1 inch. I'm sure if we went away for a few days, it
would be empty. There are no wet marks on the floor next to it, nor
are there any on the ceiling below it. Why is it going down? and how
do we fix it?

Thanks for your help!
Cheers,
Deek