Youhave not made it clear whether the water level is going down in the tank or in the bowl. the difference is important.
If the tank level is falling, water is leaking from the tank to the bowl. The only place that this can occur is through the flush (ball or flapper)valve, or (very unlikely) through some kind of siphonage from the tank into the overflow pipe.
If the bowl level is falling, and the bowl is not cracked, there are only two ways this can happen. 1) simple evaporation; and, 2) a vacuum in the waste line that sucks water out of the bowl. Item #1 would take many days or weeks to occur. Item #2 implies faulty venting.