Most often the source of the DC is the Triac, which is why
I don't use them.
Even if you don't use a Triac, there might be a lamp dimmer
or a hair dryer on the circuit somewhere that does, and
so DC blocking is often necessary to keep the toroidal
transformer's mechanical noise down.
You will see this more and more, as the lines appear to
be getting dirtier with time.
Bridge rectifiers in the AC line for blockage cannot be relied
upon that well, and the best solution is two of them in series,
paralleled with the back to back electrolytics.
Actually just a nice fat pair of electrolytics is up to the task.