I hope this is not the case or your car is still under warranty.

I had the same problem for our X3. Filled the whole thing to only have the fluid gone within a week wthout using any and no evidence of leaking on the ground etc.

Here's what I found out:
The normally competent engineers did a stupid thing on the X3s. The connection line that runs fluid to the rear window is routed thru the center console of the car. The connection in this line tends to Fail.

The fluid may be leaking onto your carpet. I got our problem covered as a Goodwill repair as we were 4,000 miles past warranty. Otherwise, it's like $1,000 repair of which like 95% is labor.

Basically, the proper repair is to Remove all the seats, remove the wood trim at the gear shift, inspect all of the associated wiring for damage, remove the carpet and padding, let them dry out for a few days and treat for mold (lysol) if needed.

Dealers have a repair kit for the break in the line, I think it's basically Glue.

Please report back on your findings.