I always try to track something else in the shot even if it is on a different plane until the object comes on screen. After I get the initial track of the object I move my tracking spline to another object that stays on screen and track it backwards or forwards frame by frame. This usually does the trick because you just want the basic motion of the camera while the object is coming/going offscreen.
You can also just parent the tracking null to another null then manually keyframe the position of this “FIX” null for the few frames that it comes and goes. But I do this less often because it’s hard to get the motion just right.
Dan Fredley