Dear all
The answer to Seans request was, that the white area in question was
defined as 0/0/0/0.00101 (DeviceCMYK)
For Preflight this are is not white, therefore the predefined rules
to check for white overprint did not find those white areas.
I had to definea new Rule that checks for light objects set to
overprint.
best regards
Peter Kleinheider