On most a/c the AP is never entirely "in trim" - there will be servos holding small amounts of torque - just as a pilot hand-flying may not be perfectly in trim, just holding a residual force.
So when an AP (any AP) disengages, it's likely the aircraft will drift slightly to the trim position. Since the AP commanded trim usually lags the aircraft behaviour, you can usually predict the direction of the drift as being a reversal of the most recent aircraft trends.