Since you dont have air suspension I can't think of any other pressure line/reservoir or any of the sort in the location you're describing. Perhaps brake lines but you'd get a warning in the dash for that. I'm baffled.

Sent from my SM-G900F using Tapatalk

__________________
BMW X5 E53, 4.8iS - 2005
Professional Stereo/Nav with DSP (modified BM54 module) - Converted to Eonon GA5166X with Reverse cam
4-Corner Air Suspension - Converted to BC Coilovers