Ok, here is an update.
I closed the intake valve and went away for a few days.

When I came back the water in the bowl was down once again.
But, the water in the tank had not moved.

Here's another piece of info.I live in a townhouse with a flat roof,
and the roofers are replacing the roof at the moment.

Does it make sense that it would be vent pressure
sucking the water out?

Thanks a lot!