With a multiball game, which most were by then, it depends on where the ball is marooned when this happens. The 'outhole' switch which it finds first after draining simply serves to kick the ball away onto the storage ramp or trough, where the relevant numbered switch shows that it's returned, and bonus award/ball-in-play advance/shoot again/player change/Game Over takes place. If a ball remains in sight in the vee of the apron, the outhole switch is at fault, if it's thrown over but nothing else happens it's a problem with a trough switch - which one depends on the state of play, i.e. whether balls are locked on the playfield.
On the next occasion, you could put the machine into test before it resolves the problem. Don't let it try the solenoid test, push past that into 'Switch Levels'. This'll list what switches are showing as closed, so the outhole (or trough) switches should be mentioned, e.g. including outhole/trough 1/trough 2 for a ball drained and the other two under the card holder/apron, or trough 1/trough 2/trough 3 for all three 'home'.
