wrote:
I flush the toilet. The tank and bowl refill fine.
[...]
Then I come back next time. And the bowl is almost empty!

Sounds like you have a clogged vent stack somewhere in the house. When
something else in the house drains (eg, a washing machine), it acts as
a venturi pump. Normally it would pull air down the vent stack into the
drain system but if that is blocked instead it pulls the standing water
out of your toilet.

Una