Speculation, but could it have to do with the investigation of the out-of province murderer who killed and injured his family in Oshawa? They are using the default NAC and the only assignment for that frequency is out in Milton.
The TG:1 tone on 421.0375 is still going. I am assuming that they are using it to map out the reception/signal quality. 423.0375 is the TX'r for OnQ network at the Baltimore site 38. Not much traffic via that site, pretty much just Ace Delivery. Every once in a while, IMOS will show up, when they are operating in the far east of Durham, when they are not using their simplex.