the better question is why didn't that show up in medical scans. could it be all this time he's avoided or faked medical scans?
Vulcans and Romulans are physiologically very similar. 3Human-1Vulcan hybrids and 3Human-1Romulan hybrids presumably are four times more similar than Vulcans and Romulans. The difference is probably subtle enough that you won’t find it unless you’re looking for it.
They probably don’t look for it routinely, because there is no law against people of 25% Romulan blood serving in Starfleet. (Saavik was 50% Romulan and served openly in Starfleet.) Tarses lied on his application in an attempt to avoid the prejudice and suspicion with which Romulans are typically regarded, not because the truth would have kept him out of Starfleet, but because the truth would have made his life in Starfleet a lot more difficult.
I still say Tarses didn't lie. Either that, or the admissions policy is bogus, with Starfleet giving a crap what race (as opposed to species) you are. Did Sisko have to check "black" on his form?
At any rate, isn't Betazoid telepathy largely intraspecies, with linguistic communication possible only in some special cases (imzadis and such) where the "protocol" or "language" of the other person's cognitive processes has been thoroughly learned?