So I hit the trunk release and I hear a swishing sound.......

I swing the trunk lid up and down violently and sure enough it sounds like there is
water sloshing around in there somehow.

Is the spoiler hollow? How could water even get in there?

No water comes out while doing the spastic trunk excercise

Could be in the trunk lid itself. It has been reported that some washers between the trunk lid and spoiler were absent when assembled and caused leaks.