The easiest way is to use a boolean or variable.

(global "boolean" x 0 )

(script startup stop
(sleep_until (volume_test_objects trig 2 (players))15)
(set x 1)
)

(script continuous hog_checker
(if
(= x 0)
(begin
(if (vehicle_test_seat_list vehicle1 "W-passenger"(players))
(deactivate_team_nav_point_object "player" vehicle1)
(game_lost))
)
)
)

If you require assisstance, just ask.