If a given recording is panned fairly hard left and right, then most of the instruments will appear to be coming from the general location in which the left and right speakers are situated. But this is really coincidental.
Disappearing essentially means non-localizable. With a full classical orchestra, the first violins should (normally) equally occupy a swathe of the half-circle in front of you that starts outside the left speaker and continues, with equal volume/intensity, to somewhere inside the left speaker.