unfortunately I think its a common gran turismo issue .water flows down the rear window/trunk drip rails and through the inside of the rear bumper (next time you wash your car if its working correctly you'll see two puddles directly in the middle underneath the car. If theres too much water going down at the same time / perfect angle (a rain storm as the perfect example) the water wont go straight down those channels and instead starts to pool in the area directly under that cushion in your picture because the actual interior trunk is plastic bolted on to the rear steel car and isnt sealed with a gasket or atleast the gasket isnt great... as soon as you take off that cushion theres 3 /4 torx bolts that I imagine are rusty since thats where the water is coming in from. (a bunch of us here have the same issue and mine is a 2015 so you woulda thought they fixed this simple defect...)
