Turns out my 3 cats thought the salty, fishy water tasted pretty good! So after one practically fell into the sump, the Mrs grabbed whatever was nearby to cover it. i.e. A chess board and the filter floss.
When I got home, I made my own custom cover out of a trash bag and duct tape!
I also moved the skimmer into the return section.
It's crude, I know. But the cat's were distraught for days trying to get in!