Just saw a newspaper article on how the Swedish Coast Guards had a busy day trying to locate two pontoons or floats that had gotten into the sea lanes on the Baltic.
Seems like an Estonian company had built pontoons 45 metres long with a freeboard of 1 metre and then sold them to norwegian fish farmers, but managed two lose two while towing them down the Baltic coast of Sweden. This caused general alarm after the local Coast Guard discovered that they could not locate these on any of their RADARs. Despite their metal construction and large size.
The reason given for this was that the RADAR beam "ricocheted off the waves" and was thus lost on the receivers (?). The conditions seemingly were only moderate, with winds around 12 metres per second max., which however will give rather steep waves with white tops all over in this brackish (low salinity) body of water.
This would seem to indicate that wave conditions may well be the most important single factor effecting the visibility of low objects on RADAR. Those pontoons had an awful lot of metal in them, so the lesson for kayakers should probably be that no extra inconvenience from RADAR reflectors is worth the trouble as soon as one is among waves high enough to obscure the horizon even occasionally.
So the skeptisism expressed by many in the earlier thread on this subject is well supported by the report on this incident.