Locking gas cap?
 

Forgive me if my question is too stupid or smells of a newbie.

I am seriously considering buying a X3 in the next few days. At the dealer, I opened the gas flap door on a X3 whose doors were locked. Is the gas cap exposed and not lockable?

No I believe it locks with central locking.

Another neat idea I have discovered on my 08 x3 2.0d is that a valve remains shut in the filler neck until the derv nozzle diameter forces it open. This being larger than petrol nozzles [in the UK] and prevents mis fuelling. It probably also prevents theft to all but the most determined.

Look at the manuel, (see this site for the right one) I think the gas door lock is timed delayed.