What is this "missing security cable"? I recently purchased a Bell Overpass 200. On the outside of the box it says, "Warning: User must secure bicycles to carrier with additional security cable, not included. See the enclosed owners manual for proper installation and use of your bike carrier." Four rubber straps are included, though not mentioned on the box. There is no owners manual included, just the setup and takedown instructions.

edit  This is a car boot-mounted bike carrier
It sounds to me like your four rubber straps are to hold the bikes together more tightly unless they're used to clamp the frame to the carrier.

The missing security strap is an anti-theft cable. Robberies from the backs of vehicles seem to be on an increase and any deterrent you can put in place to prevent theft is worthwhile.

In this case, the manufacturer is deferring the anti-theft security over to you so they are not liable if your bikes get stolen f I'm the rack.

When you're mounting your bikes, it's worth using some old rags or similar to help prevent rubbing and scratching in-transit too.