The Flying Squad by Edgar Wallace

To give him what credit was due. Mark McGill had, in a half-hearted way, endeavoured to minimise the risk which Ann so often ran. In nine cases out of ten she carried an innocuous package of common salt in her journeys to the country. The real "cargo" was hidden in a specially constructed cupboard let into the side of the car and concealed behind the leather lining.

Ann had certainly carried a dangerous load to Oxford that night, and he trusted her native wit to get rid of it if there was the slightest danger of detection. In the presence of Bradley, he recalled an alarming circumstance. A week before he had sent her to Birmingham, and the hollow side of the car had contained a fairly large quantity of cocaine.