it's only happened the last few days for me and it's been warm. Also I checked under the hood and there doesn't seem to be a resevoir visible for PS fluid. I see windshield, antifreeze, and motor oil. Nothing in the manual's index either.

For the E53, I found this thread which says it's marked under the ATF cap (transmission fluid???): http://www.justanswer.com/bmw/1etse-...d-x5-2004.html

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr