i dont know which year you have but look into this first:

Summary:
BMW IS RECALLING CERTAIN MODEL YEAR 2012 X5 VEHICLES EQUIPPED WITH A DIESEL ENGINE AND 2013 X5 AND X6 VEHICLES. DUE TO A MACHINING ERROR OF THE STEERING GEAR SURFACE, POWER STEERING FLUID MAY LEAK.
Consequence:
LEAKING POWER STEERING FLUID CAN SPRAY ONTO HOT EXHAUST PARTS, INCREASING THE RISK OF AN ENGINE COMPARTMENT FIRE.
Remedy:
BMW WILL NOTIFY OWNERS, AND DEALERS WILL INSPECT, AND IF NECESSARY, REPLACE THE STEERING GEAR FREE OF CHARGE. THE SAFETY RECALL IS EXPECTED TO BEGIN DURING JULY 2012. OWNERS MAY CONTACT BMW CUSTOMER RELATIONS AND SERVICES AT 1-800-525-7417.

second place is the reservoir which is typical of bmw and lastly the lines (pressure hose and housing). if it was the reservoir it just have been build up which was cleaned off with a can of brake parts cleaner and has not built up again which would explain a vanishing "leak".