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.