Here is what is written in "Fighters over Tunisia":
"
Around this date Oblt. Schiess of III/JG 53 had also apparently attacked and claimed a B-17, which was not confirmed, though he had to wait some time to find out whether confirmation was likely to be forthcoming. On this date 24 B-17s had attacked a convoy off the Lipari Islands, just to the north of Sicily; one B-17 was very badly damaged during this attack, and subsequently crash-landed at its base. It is possible that this was the aircraft claimed by Schiess, but details to confirm this are not available."
At this date, it seems to me that all B-17 units in MTO were based in Algeria. So to attack a target north of Sicily they may well have flown close by Cagliari, even if they did not attack it. And the above mentionned B-17 may have been attacked by Schulz rather than by Schiess.
Note: the 301st BG website (
www.301bg.com) reports that the mission this day was "off Palermo", so consistent with an attack on shipping north of Sicily, but has no more details.