Iodine-131 has a half-life of ~8 days. But Iodine-131 has been detected for weeks "Across Europe" indicating that the release was not a single event. So either a nuclear reactor is leaking (?), organized crime has gone into the medical radioactive waste disposal business, or some (inept) bad actors are at work.
Governments typically mandate that nuclear power plants collect and analyze environmental samples from the area surrounding the plant on a periodic basis. I therefore find it hard to believe that a European nuclear plant is leaking. Russia/Ukraine?