could it perhaps be that, at least in E:TW, certain units (like native indian units) are invisible as they should be, whereas others (like pandours, rangers) are only hidden from the A.I. at easier difficulty levels?

if that's not it, then I cannot explain why some people have the A.I. seeing their supposedly 'hidden' units, whereas others have them behave like they should (at least in regards to hidden units)