Resurgence of people using third-party programs to find hunt marks?

[Vivi_Bushido]

The term you look for is called "Radar".

Simply said S Ranks and several FATEs have a extremely huge draw Distance, so if you stand on the edge of the Zone Map, you still will see the S Rank being on the other side of the Map.
Now what those Tools do is basically scanning the Memory ID of the S Ranks / FATEs and poof.

These tools do exist and players use them all the time. In fact, ACT has a feature on it that does the same thing with the use of triggers. These triggers are posted on FFXIV reddit and most ACT users use them. (Parse programs can do a whole lot more than parse). From the description of the triggers, it calls out any hunt regardless of rank when you reach a specific amount of yalms to the target. It's most probable that ACT triggers are the radar that these suspicious individuals are using.