I don't know how true it is or not, but I read in several places that ships are not obliged to monitor AIS, besides to have it available on a small screen. But they are obliged to monitor radar, and for larger ships radar is recorded for eventual investigations, so special attention is paid to it.
In the meantime, John from Echomax said mounting the Active-XS unit near to the side of a mast is a bad idea. It must either be on top of a mast or suspended between spreaders or backstays.
Martin, I think a good place for passive radar reflectors is above the radar on the mizzen mast, about mid-way between radar and top.
SM #151 Travel Bug