Other than sunroof drains, the only places that come to mind would be the passenger door window seal, the door seal at the bottom, and maybe some other leak (e.g., gasket) on the sunroof (this type of leak would likely stain the headliner).
Have you had water in the passenger front footwell before (I realize you now seem to be talking about the rear footwell)? I’ve seen reports of the drains actually becoming disconnected in the area near the passenger footwell, in which case blowing air through wouldn’t solve anything.