USSR tried to keep Chernobyl blowing up a secret until caught and preassured by Sweden to admit there was an accident. I really don't expect Russia to act different today. Just look at the "we have nothing to do with it" handling when Crimea was invaded.

The Communist government even tried to hide the blow up from local population exposing a lot of people to extra risk when trivial measures could have minimized it.

In Minsk, Belarus my mother was warned by friends of friends that worked at a research reactor. They spent few hours trying to identify a leak at their facility as all Geiger counters showed a very high rise over the background level.

Then they were reluctantly told that it was an accident in Chernobyl. Then from the reading of the counters they realized how bad the accident must be and asked all people they knew to stay in doors and close all windows.

On one hand, 100 TBq is less than a thousands of a gram of Ruthenium 106. On the other hand, anybody handling radioactive samples with terabecquerel activity will be very very aware that they are handling a lot of radioactivity. So my guess is, either an accident or an breach of safety protocols at a nuclear facility. Either way, the source is most likely aware that something did go wrong.

It's quite possible the source is aware but it's being hidden on sub-government or institutional level. Not uncommon in places with personal responsibility for public or corporate accidents.