Eugene,
Like the others said, it is most likely the sunroof drain. I have the same issue in my driver side footwell. I tested it by pouring water down through. I have the disconnected drain tube issue I believe.
I actually have yet to fix it since it hasn't been a HUGE issue since I figured it out. Although, "Poor Man's Fix" until you get a day off to fix it. Park with the nose of your X higher than the back. This way it'll drain out the back. Seems simple, but it works for me.
Anyway, best of luck!

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2004 E53 X5 4.4i
Black Sapphire/Dakota
Cold weather / Pano sunroof
Built: 07/2004