In traditional Greek practice, from what I have gathered, a retired bishop returns to the monastic state and never serves again...the Russian practice has retired bishops serving if they have a synodal blessing. Greeks in America seem to do this. Of course, even in traditional practice, one can be "unretired." So I guess the practice has always been a little fluid.
I don't know about "never serves again." Most of the retired bishops of the GOA in this country do indeed serve after they've retired: +Anthimos, +John, +Iakovos (when he was alive) have all served while they were healthy enough to. +Gerasimos of Abydos served in his retirement as well.
When one becomes a bishop, their obedience to their abbot is broken (their obedience is now directly to the Synod); I'd agree with the assessment that they return to monastic life, though - isolation, prayer, daily work, etc. But they don't necessarily return to a monastery per se, and if they do, they're normally considered a "guest" and not under obedience to the Abbot.
(As an aside - one could actually argue that no one of the Great Schema should be made a bishop, since they are bound to obedience to their Elder, and a Bishop cannot be bound as such to another man.)