Good that you found that. Now keep it filled and hopefully you can figure out where it is leaking.

I replaced the o-ring on the cap on mine while I was tracking down the easy stuff after first getting my car.

Can't find it now, but here is the full cap, which I assume includes the o-ring. ZF for about $3. PN 32-41-6-851-332

https://www.rmeuropean.com/Products/...32-MFG103.aspx

EDIT - found the part number I used when I bought just the o-ring:
32-41-1-128-333

Here's that for $6 at ECS tuning:
https://www.ecstuning.com/b-genuine-...l/32411128333/

Seems like the cap costs negative $3 right now. I'd buy as many of those as I can.

FYI - When I bought my X5 8+ years ago, I did a "change every fluid in every compartment" thing - the appropriate Redline everywhere, including P/S fluid.

High presssure lines replaced, slight leak in P/S pump and I only have Castrol ATF this time. I noticed a noise when cold, as it was winter. I then evacuated the system and refilled with Redline P/S fluid - no noise even at -20 C.