I may have found the answer! I just spoke with a business associate, and he had the exact same thing happen to him, not once, but TWICE! He experienced one toilet bowl water level fine, while one lost water - they were on the same floor! Another occasion, he said the toilets upstairs were fine, and the main floor toilet bowl lost water. He said in both instances, there was blockage down line. Once he cleared the blockage, the problems were resolved. Do you think this is purely coincidence, or is he on to something? He couldn't explain it, but said once the blockage was cleared, the toilets were normal again. What do you think?