Under what circumstances would your employer ever consider searching your car if you do not sign the form or ever bring it up to them?
Carrying is blatantly breaking policy, but discreetly storing your firearm in your locked car - inside the holster, seems like the smartest, safest option.
Hopefully your state laws will change. Recently it became law in my state that an employer could not regulate keeping firearms in locked cars parked on premise. To disarm you while you are working is one thing, but to deter you from exercising your rights to and from work is even more ridiculous.