I also recently noticed Union City is no longer active on the published TGID's.
Never had a problem in past picking up any of these cities including Fremont & Newark.
I can still monitor Fremont & Newark, but Union City is not there.
For what it's worth I did some logging and did pick up a few radio checks/mic tests a couple weeks ago, on the main dispatch TGID.
So sounds like they are doing maintenance of some type.
Maybe they are temporarily using a different TGID or have moved to a new set of TGID's altogether.
As far as I know they are still on Alameda County's 800 system.
Also, there is still activity on the City's Admin TGID's from the Buses, Public Works, etc..