from BBC News Asia
... Commander William Marks from the US Seventh Fleet, which is taking part in the search, says he expects the plane's flight recorders to be floating in the water.
"In calm seas, if there were a soccer ball [football] or a basketball floating in the water, the radar could pick it up. They [flight recorders] typically have a radio beacon and so for example our P3 [radar] - if they are flying within a certain range of that - will pick up that radio beacon. We have not yet picked up anything, but that's typically what those black boxes contain." ...
The sentence in bold letters, is incorrect, as far as my knowledge is concerned.