No overprint objects but a dash appears in Attributes panel
Documents that have no fill or stroke objects set to Overprint show a dash in the Attributes panel checkbox that makes you suspect there is overprint. Confusing, nothing should be checked if nothing is set to overprint. The example show a dash for fill when the image is selected and a faint dash for stroke when the text is selected.
Hard to grasp and confusing.
The behavior is not new.
For objects without a fill the Overprint Fill is considered mixed, and for object without a stroke the Overprint Stroke is displayed as mixed — as Kurt says on forums. CS6 does the same.
I am not really sure why Ai does it, really, but even in Ai 10, before 'mixed check states' (minus) got introduced, we had these as 'shallow' checkmarks. Perhaps to mark a 'potentially ambiguous' state?
However, why an image with a forced stroke gets a mixed Overprint Fill, and an active one... the only guess — the image still has no fill, but Ai had to add an empty appearance attribute for it when its stroke was forced. Is still has no fill, thus it’s undefined, but the attribute is forced, thus its non-disabled.
It makes somewhat sense. Hard to grasp though.
See also this forum discussion: https://community.adobe.com/t5/illustrator-discussions/overprint-attributes-panel/m-p/14110657/page/2#M381782