Radiation detectors can detect very low levels of radiation (far below any measurable health effects, for instance), so claiming we can still detect fallout from Chernobyl doesn't really say anything.

To quote from the article I linked to:

> In the last years values of up to several thousand becquerel per kilogram were measured in wild game and certain edible mushrooms. In Germany it is not permitted to market food with more than 600 becquerel caesium-137 per kilogram.