So far, we haven't been told whether Dahj-type advanced synthetics are distinguishable from ordinary wetware or not. Might be there are bits and pieces of machinery inside, including some that send positronic signals. Might be those will show up in a scan or an autopsy, neither of which was performed on Dahj. Might be scans are blocked by jamming. OTOH, Dahj activated when threatened; if we assume Vulcans or Romulans or Hungarians or left-handed people are advanced synthetics, then this fact remaining secret might be due to them always turning into killing machines when exposed, and eliminating all witnesses. And then perhaps going passive again.
A Vulcan health check might be interesting to watch with an unjammable Mk I Eyeball, then: the Doctor goes "Hey!", the Patient assumes the tal'shaya position, the Doctor likewise springs to action, then both go "Ah..." and return to normal, with the Doctor signing a clean bill of health (even if with some reservations about that overindulgence in sandweed, and a kind reminder to Mrs Patient to pay a visit on schedule, too).
Timo Saloniemi