I'd side with new construction. There are several projects there along W Main and around the center of town, and the female on today's testing asked about fire alarm testing.
As for the 46XXX radios, those were definitely handed over to other agencies. 54EMS has ~2 portables, one being 46094. 46104 being used today is an FM18 radio. That would make sense since FM18A was the outside/lobby receiver with RID 1167 during testing yesterday and has 46104 assigned to them too. This is not specific to Somerville- Raritan's FM16 has a 46XXX unit that is slated as a Raritan PD unit, Bound Brook PD's OIC/Chief used a radio to communicate with FD on Fire Region 4 at one point with pre-arrival updates, and another non-LE agency has units that came from their PD as well.