I have never locked it up. Lived in MI my whole life, we always just cased it and put it somewhere in the gun where nobody can get to it. Ammo separate.
Hmm, so it doesn't even have to be cased.
Remember DNR has regs too, I am not familiar with them...
EDIT: DNR says cased or in trunk, better safe than sorry...
DNR - Transporting Firearms, Crossbows, and Bows and Arrows