That is very interesting, I've been closely tracking RTD TG's for several months and I've never seen a Pro96Com Affiliation from 10083, yet alone any voice grants (I do have it programmed in my scanner as an "unknown" RTD TG). I wasn't even aware RTD used the Weld Simulcast site, I guess that would be for certain parts of the Boulder-Longmont area.
It could be disgruntled/ex employees of one of their many, many contractors. My impression is that RTD limits distribution of portable radios to a relatively small group of people (supervisors, security and mechanics). However, one would think it would be fairly difficult to steal mobile radios, let alone operate them in a "stand-alone" fashion. So who knows...
-Nate