I've just come back from reading the first three episodes of Execution, and here's what I noticed:
Yura is incredibly good at detecting anomalies.
There's no way Yura wouldn't have been able to tell what Dimitri was, because Dimitri's power is just that strong.
Here's my theory on this.
Dimitri flew under Yura's radar at first, because Yura himself said that serious anomalies over the age of twelve is extremely rare.
And Dima? Dima is 16 in canon. He looks nowhere under 12 and with what Yura believes, the public also unlikely to think that Dimitri is an anomaly. Yura most likely singled out Katya earlier on and was more quick to confront her because she still looks relatively close to 12 years of age.
(From KT's Official Guide to Coolness)
And since he's found out that carrying around anomalous effects also causes his "Stalker Senses™" to go off (mentioned in the Execution comic), he doesn't believe that this 13-something year old would be carrying one around on her own volition.
In his mind, the most likely theory would be that she IS the anomaly.
But Dima is a special case, because he's 16. And 16 year olds can become Stalkers, like Sergei, and eventually Yura, did.
(Yura noticing that Dimitri has the "anomaly aura", from Occam's Razor)
He has to double check instead of directly confronting Dima himself, even going as far as to asking Sanya to experiment on the Cube™ for him just to be sure. But I do believe there's a reason why the video where Yura finds out Dimitri is a mutant is called Occam's Razor; which is a principle that says that if you have two competing ideas to explain the same phenomenon, the simpler one is often correct.
Yes, it may not be common for anomalous children over 12 to exist in the town, but it's not impossible. Therefore, Dimitri must be a mutant, in Yura's mind.
Sorry if this isn't structured well. I just wanted to get it off my chest.