Three people including a 15-year-old girl were found in separate cars at the Beni-Enzar border between Morocco and Melilla on May 24.
Two hid in special compartments behind the dashboard and a third was under a rear seat.
Two were treated by medics for asphyxia, disorientation and joint pain.
A fourth person was also found on May 24 under a truck in the Melilla port.
In recent years, thousands of migrants seeking a better life in Europe have tried to enter Spain via its two enclaves in North Africa, Melilla and Ceuta with some scaling huge fences topped by razor wire.
In January, a woman was caught smuggling a teenage African migrant hidden in her suitcase into Ceuta.