I had a similar scenario when my dad decided to move the filter so he could
better hear the splashing of the water to a higher level on the rock boarder
around the edges. Sure enough, that night (while i was away on the job in
texas) as my sister was coming home from work, she noticed less than 2
inches of water left in the pond and a dry-sucking noise coming from the
filter.
Thankfully, she wasn't too wasted to have the intelligence to turn off
the filter and go back inside to sleep as it was beginining to downpour. I
was never so thankful for a torrential downpour as i was that night. When i
came home that weekend, the fish seemed fine, and the water was nice and
clear. My dad knows not to mess with the pond anymore too.