Kind of a strange location for an auto-fill. Usually auto-fills are placed in areas where the water is still. That's why auto-fills are usually in their own canisters next to the pool. The skimmer throat has the most water flow.
Where is the source water inlet? Are you sure this isn't an attachment that had a bar across it to keep the pool cleaner from getting caught in the skimmer?

Call the owner and let them know that THEIR pool is broken.

That is not an auto fill.
they may have added a battery operated remote auto fill.
But the battery operated sensor is missing if that is what it is.
The other end of it would be at the pool equipment with a solenoid valve hooked to a water supply.

Could be Fiberstars wireless waterlevV. I don't know if there still @, the unit @ Eq should be blue & white with a hose connection. Goodluck and plan on never ending adjusting, then to the trash.