Phantom Windows 01 X5
 

What would cause the window in an X5 to open periodically on its own? I close it and its fine for another couple minutes then goes down again.

Only time I saw it happen was when somebody sat on the key fob. Holding the unlock button down opens all the windows. If the remote is fine, presumably a fault in the system could cause it to think it is receiving a constant unlock signal, causing the same symptom.

Window Regulator possibly

Ditto above...if it's not due to someone accidentally activating the convenience open feature on the remote key...its more than likely the window regulator.

The same type of thing happened to my front passenger window in my 99 540iT just before the window regulator gave up the ghost. It would lower itself a few inches...I'd raise it up...then over the course of a few days...or driving over rough pavement...the window would drop again.

Does it go down all the way? Does it act as a ghost? Could be a false ibus telegram telling it to so... Naturally, assuming that there is no keyfob involvement nor weak regulator

I have this too! My rear pass side window powers all the way open. It's random and seems to only happen upon starting the engine. I expect this is why the previous owner had removed the window switch in the door as this only started after I put the switch back in place.