The a/c may not have had a 406 as it was privately owned. A 406 signal is almost instantaneously acted on whereas with a 121.5 it could take a long time before anyone first hears it, decides if it is a false activation, figures out where it might be coming from, tries to call around and see if there are any missing a/c, etc, etc. 5 hours passing before they get the ball rolling with 121.5 only is unfortunately not uncommon.