For v3.2, I have simplified Hidden PDCs considerably. All PDCs start out hidden and cannot be detected. Once they fire or use active sensors, they lose their hidden status and can be detected normally. This is over-simplified because it means races that didn't see them fire can now detect them but it makes life much easier. I guess we can assume that firing leaves obvious traces.
An attacking ground force now has a chance to detect hidden PDCs equal to its attack strength, with a maximum of 50%. This is checked every 5 days, after ground combat. Also, only PDCs that contain ground forces can prevent a surrender.
Steve