The Coast Guard does have RDF's (radio direction finders). Back in the "old days" boats and ships had these, some still do. RDF's do not work by triangulation but by phase amplification and cancellation using a special type of antenna
. The Coasties also have portable RDF's which means that after they have a good idea of which marina or area that a bogus Mayday is being broadcasted from, they have the ability to walk around a marina for example and to nail the specific boat the distress is being broadcasted from.
If you ever hear a fake mayday, do not let on that you know it is fake. The best thing to do is to just stay off the radio
. The Coast Guard already suspects it's fake and wants very much to bust them. Don't interfere. The Coast Guard will have the perpetrators key their mike for as long as they can which enables them to better figure out the location of the transmission
using their RDF.