I just experienced it. Had a formation of B-17s attacking A19 (Stavanger, Norway) out of A1 (Norwich). Was attacked by a Ta 152 (Stepsis); he had no icons. A low P-47 climbed up and attacked but he DID have icons.
Airjer joined me as gunner, he reported no icons as well, and we chatted with Stepsis afterward, he said he didn't have icons either.
We had scattered cumulus clouds at about 15,000' and that is around where we were flying.