So for obvious (competitive) reasons, Industrial isn't releasing a full list of TGs and Users to us with scanners.
All of what you see on Scan New England has been 100% deduced by research from monitoring, hence why so many question mark TGS.
The system is so large and covers such a large geographic area, that the number of users is very high and it's not uncommon to have only very occasional active talkgroups that lead almost no information as to the user. Most of what is here in RR has been submitted by the same that do the discussion at SNE.
Back to the research part, I'll give a brief example. I heard a garbage company indicate that they are getting the dumpster behind a local pizza shop to me. So when I went to get coffee that morning, I drove by the pizza shop and saw the name on the dumpster. Turns out it matches other TGs used by the same user, IDed too.