the gas tank is baffled and under the rear bulkhead, when the tank is full do you hear the sloshing?

have you removed the rear seat bottom or center armrest cubby to check, maybe a stuck half empty water bottle.

water IN the spare tire? water in the sunroof channel? water in the first aid kit in the rear parcel shelf? battery low on distilled water? have you made sure the drain holes on the bottom of the doors are clear? is the dran hole for the trunk clear - should be a rubber tube three inches long hanging down some eight inches beyond the rear passenger side tire.

have you probed with a flashlight into the rear bumpers from underneath or even inside the top of the wheel wells - never know if someone is playing a prank