Resolved. The light in question is the 'D/S Security' light and it is just burnt out.
My confusion stemmed from the operation of the porch lamp. It seemed that
turning the security sw on or off changed the way the light detection and motion sensor
of the porch switch worked.
Further testing suggests that turning the porch off and on again changed the way the light operates.
(enabling or disabling the motion/daylight sensors)
If that is NOT the case I don't mind being corrected.
Ragards,
Bob