One thing I can think of is you might have a venting issue and the water gets partially sucked out of the bowl trap when other fixtures are being used or when a negative pressure situation occurs in the main drain line. The only other thing I can think of is a pet dog likes to drink from the bowl.
I've noticed that the water level in my toilet bowl goes down about 1 inch per day when the toilet is not used. The floor is not wet around the base of the toilet. What could be the cause?
You might have a hairline crack between the water trap in the toilet and the drain. One toilet in our home had a hairline crack in the shell which was very difficult to see. There would be water on the floor and I removed and reinstalled the toilet twice thinking the wax ring was leaking. When I finally found the hairline crack it explained everything.