Doc here,
Immobilizer , Keys or not..
Any thief can simply jump battery power to the BATT terminal on your car (Or in most cases, jump a wire between the IGN and BATT terminals at the fuse box) and a jumper at the "S" and "I" terminals..and he's off and running! takes 30 seconds for two to do.
I can give you a relay operated system with several options that is easier that that one if you like..BUT remember: Nothing is foolproof..whatever the mind of man can conceive, the mind of man can defeat..
Doc