Sorry I missed your earlier post asking how much I paid for the bulbs in the high level stoplight. I think they were about £1 - £1.30 each from the main Alfa dealer. Something like that anyway. Not cheap, but while I had it in bits I decided to change them all. The dealer sold them individually, not as a complete set. The ones that were working had gone a funny silver colour and looked like they were about to blow anyway.
As other people on here have said, the high level stoplight does not affect the bulb warning indicator on the dash. I had 3 bulbs out in this stoplight and the warning light didn't come on. Even removing the 3rd stoplight assembly completely doesn't cause a warning. (Note that my car is a 1998 S reg - I guess they might have changed since then).
My guess is you have an intermittent connection on a rear number plate light. Pop the plastic holders out of the bumper, turn the lights on and rough them up a bit to see if they flicker. Definitely a weak point on the 156. If one flickers, turn off the lights, remove the bulb, spray contact cleaner in the holder, replace bulb with a good wiggle and retest.